As he said.
A useful tool for spotting them - in later versions of Windows - is the Task Manager. It can with most versions be accessed by pressing Ctrl-Alt-Del all at once (and once only!).
Then pick "Task Manager". Click on "Processes".
If you click "CPU" it will show you which processes are using most processor capacity, or on "Memory" ditto for memory. The click toggles from "largest first" to "smallest first".
If you see one process using more and more memory, that's one to suspect.
Every so often, I look at the list of what's running and stop the more obtrusive ones. MSCONFIG will allow them to be disabled - if they put themselves back after that, they usually get uninstalled. You do need to be a bit cautious doing that, not to remove stuff that is essential for running the machine. I find Apple and Google particularly keen on running all kinds of stuff without being invited. I've removed about 5 GoogleUpdaters in the past couple of weeks.