Why is my desktop PC running slow?
This is the single most common cause of a slow PC, and most people have no idea it's happening. Every time you install software, there's a reasonable chance it has added...
Tag Slow Performance
Why is my laptop running so slow?
Open Task Manager (Ctrl + Alt + Delete), click the Startup tab, and have a look at what's in there. Anything you don't recognise or use regularly can be disabled...