Updated July 2011
A good, simple thing to check on ALL you computers is the startup folder. To see what programs start up every time you start the computer, go here:
Windows XP–Click Start > Programs > Startup
Windows 7–Click Start (the weird icon in the lower left corner of the desktop) > All Programs > Startup
If you see programs here that you don’t need to start up when you start up the computer, right click on the program name and select “delete”. This doesn’t delete the program, but makes it not part of the startup. If you are feeling really adventurous, you can also use CCleaner to see further detail on programs that startup on their own. Be careful with this, of course – if you don’t know what a program does, don’t disable or delete it until you do more research.
Also, the MakeUseOf site has this wonderful “Guide to speeding up Windows” for you to download:
http://www.makeuseof.com/tag/download-windows-speed-free-pc-tune-guide/