Security Task Manager
If your PC is sluggish, there's a good chance you've got far too many programs and services running, many without your knowledge. And for all you know, some of them may be malicious. Running the Windows Task Manager helps, but only slightly, because although it shows you what's running, it doesn't show many details about what's there.
This program gives a wealth of detail about each program and process, including whether the program is likely malicious or not, and it lets you delete any program and process with a single click. You'll get an enormous amount of detail about what's running, including the manufacturer, type of program, how it started (for example upon startup or from within Windows Explorer), and the file name. It also rates files according to how harmless or malicious they may be. To stop a program, highlight it and click remove, and you're done.