- Disable the useless programs that open right when you start Windows (or any programs you wont immediately use)
- Empty your "Temporary Internet Files" folder
- Empty your "Temp" folder
- Empty your "Cookies" folder
- Go to download.com and find a cleaning utility that finds outdated/bad files like uninstall files to programs you've already deleted, etc.
- After you've done all of the above, restart it, then use the disk Defragment utility (you'll have to let your comp sit while it defrags)
And
- Restart your computer at least once every day or two
If you have $$$
- Invest in more RAM
What operating system are you running (Windows, Mac)? How fast? Is your hard drive nearly full?