Even though we've been working with Microsoft products for over 20 years, sometimes very strange things happen on our perfectly configured computers.
Here are three things we look for immediately to speed up a system:
1) Remove all unnecessary programs that are running the systray (the systray is all the little icons in the bottom right next to the clock). You can turn them off permanently by going into msconfig.exe and unchecking the programs that shouldn't be there. Just be careful with what you are unchecking.
2) Uninstall programs that you will never use again. Go to Startup>Control Panel>Add/Remove Programs (only on Windows XP).
3) Finally, AFTER unwanted programs are removed, clean out your Windows registry by using a registry cleaner. We recommend RegCure.
As always, make sure your virus scanning software is up to date and run full scans once per week. We recommend AVG - the free version works just fine.