A virus doesn’t usually announce itself. It just quietly makes things worse – your computer gets sluggish, something pops up that shouldn’t, or a program starts behaving strangely. By the time most people suspect something’s wrong, the infection has been sitting there for a while.
Here’s what to actually look for.
Not every slow computer has a virus
Before getting into the signs, it’s worth saying this clearly: a lot of the symptoms below have perfectly innocent explanations. A slow computer might just be a slow computer. Pop-ups might be coming from a dodgy website, not malware. An unfamiliar program might be something Windows installed without asking.
The signs below are worth paying attention to, but none of them in isolation means you definitely have a virus. A cluster of several at once is when it starts to look more likely.
Your computer has slowed down suddenly – and there’s no obvious reason
Gradual slowdown over months is usually just age and accumulated clutter. But if a computer that was running fine last week is suddenly sluggish, struggling to open programs, or taking twice as long to boot, something has changed. Certain types of malware – particularly cryptominers and botnets – quietly take over your processing power to do things in the background that have nothing to do with you.
The difference to look for is sudden versus gradual. Sudden slowdown without an obvious reason – no new software, no Windows update – is worth investigating.
Pop-ups are appearing outside your browser
Pop-ups inside a browser can come from dodgy websites and aren’t necessarily a sign of infection. Pop-ups appearing when your browser isn’t even open are a different matter. So are pop-ups warning you that your computer is infected and urging you to call a number or click a link immediately.
That last type is almost always a scam. It’s called scareware – fake security alerts designed to frighten you into doing something. Don’t call the number and don’t click anything on the pop-up. Close it through Task Manager if you have to (Ctrl + Alt + Delete, then End Task). Real antivirus software doesn’t work like that.
Your browser homepage or search engine has changed
You open the Chrome or Edge app and you see something you’ve never seen before. Or searches are going somewhere other than Google. You didn’t change any of this.
This is a classic sign of a browser hijacker – malware that gets in through a software download or a dodgy extension and takes over your browser settings to redirect your traffic. It might look like a legitimate search engine but it’s serving ads or worse.
Programs you didn’t install have appeared
Open your list of installed programs (Settings > Apps on Windows 11) and check for anything you don’t recognise. Some of these get bundled with legitimate software downloads – you tick through an installer quickly and end up with an extra toolbar or utility you never asked for. Some are more malicious.
If you find something unfamiliar, search the exact program name before deleting it – occasionally Windows installs components with odd-sounding names that are actually legitimate. But unknown programs are worth checking.
Your antivirus has been disabled – and you didn’t do it
Some malware specifically targets security software first, disabling it so it can operate without interference. If Windows Defender has been switched off and you didn’t do it, or if a scan that used to work is suddenly throwing errors, that’s a significant warning sign.
Files have gone missing or you can’t open them
Missing files can have a mundane explanation, but if multiple files are disappearing, have been renamed, or you’re suddenly unable to open documents you accessed recently – especially if accompanied by a message demanding payment to restore access – that’s ransomware. It encrypts your files and locks you out until you pay. Don’t pay. Bring it in immediately and see our Virus and Malware Removal page for more on how we handle ransomware.
Your contacts are getting emails or messages you didn’t send
If someone tells you they received a strange email from your address, or you find sent messages you don’t remember writing, something has access to your account. This might be a compromised email account rather than malware on the device itself, but either way it needs dealing with quickly – change your password from a different device as a first step.
The fan is running hard for no obvious reason
If your computer’s fan is working overtime when you’re not doing anything demanding, something is using your CPU heavily in the background. Open Task Manager (Ctrl + Alt + Delete) and look at the CPU and Memory columns. If a process you don’t recognise is sitting near the top chewing through resources, that’s worth looking into.
What we actually see come through the door
Most genuine virus cases we see fall into a few patterns.
The most common is adware or a browser hijacker that came in bundled with a free download – a PDF converter, a video player, something like that. The person installs what they wanted and three other things come along for the ride. The computer starts showing ads, searches go somewhere weird, and a toolbar appears that nobody asked for. It’s annoying but usually straightforward to clean up.
The second pattern is scareware – the fake virus pop-up scenario described above. Someone sees the warning, panics, calls the number, and ends up with genuine malware installed by the “support technician” on the other end of the line. By the time they come to us, there’s often a remote access tool sitting on the machine. If this sounds familiar, change your passwords from a different device and bring the computer in promptly.
Ransomware is less common in home users but it does happen. It usually gets in through a phishing email – someone clicks a link or opens an attachment that looks legitimate. The first sign is typically files that suddenly can’t be opened, followed shortly by a message on screen.
What to do if you think something’s wrong
First, don’t panic and don’t start clicking on things trying to fix it yourself. Some malware is designed to spread further or delete itself if it detects you trying to remove it.
A few sensible first steps:
- Stop using the computer for banking, email, or anything that involves passwords until you know it’s clean
- If you have antivirus software, run a full scan – not a quick scan
- On Windows, open Task Manager and check what’s running. Anything sitting at the top of the CPU or memory list that you don’t recognise is worth noting
- Don’t try to uninstall unfamiliar programs manually if you suspect a serious infection – some reinstall themselves or cause damage when removed improperly
If a scan comes back clean but the symptoms are still there, that doesn’t necessarily mean the machine is fine. Some malware specifically evades standard antivirus tools. Bring it in and we’ll go deeper.
Common questions
My antivirus says the computer is clean but it’s still acting strangely. What does that mean?
A clean scan result doesn’t always mean a clean machine. Some malware is specifically designed to hide from or disable standard antivirus tools, and some infections leave behind components that continue to cause problems even after the main threat is removed. If the symptoms are still there after a full scan, bring it in. We use tools that go beyond what consumer antivirus software checks.
Can a Mac get a virus?
Yes, though Macs are less commonly targeted than Windows machines. The idea that Macs can’t get viruses is outdated – adware, browser hijackers, and more serious malware all exist on macOS. The signs are largely the same: unexpected slowdown, browser changes, strange pop-ups. If something feels off, it’s worth checking.
Could it just be a slow computer rather than a virus?
Quite possibly, yes. Gradual slowdown over time is usually dust, an ageing hard drive, or startup bloat – not malware. The things that point more toward a virus are sudden changes, browser behaviour being hijacked, programs you didn’t install, and pop-ups outside the browser. If you’re not sure, a health check is a good place to start – we’ll look at both possibilities.
Is it safe to keep using the computer if I suspect a virus?
For general use, probably fine in the short term. But avoid anything sensitive – banking, email, anything involving passwords – until you’ve had it checked. Some malware is designed to quietly capture that kind of information, and you won’t necessarily know it’s happening.
How do viruses get onto a computer in the first place?
The most common routes are email attachments or links, software downloaded from unofficial sources, and browser extensions or free tools that come bundled with extras you didn’t ask for. Keeping Windows and your browser up to date closes a lot of the doors malware uses to get in.
Buzzline
If your computer is doing something it wasn’t doing before and you can’t explain why, it’s worth getting checked. Most infections are fixable, and the sooner they’re caught, the less damage they do. Give us a call on 03 9822 3481 or head to our Virus and Malware Removal page to book a time.
Sources
- National Cybersecurity Alliance. “How to Tell If Your Computer Has a Virus and What to Do About It.” staysafeonline.org/articles/how-to-tell-if-your-computer-has-a-virus-and-what-to-do-about-it
- US Federal Trade Commission. “Malware: How To Protect Against, Detect, and Remove It.” consumer.ftc.gov/articles/malware-how-protect-against-detect-and-remove-it
- Norton. “How to Tell if Your Computer Has a Virus: 11 Warning Signs.” us.norton.com/blog/malware/how-to-tell-if-your-computer-has-a-virus