I am assuming you use Windows XP as your operating system?

Well, I would clear up temp folder which usually contains all the temporary files used when installing applications, games, etc. That folder is located in C:/Documents and Settings/(Your Computer Name)/Local Settings/Temp. You would be surprised at how much space these temp files are taking up.

Clear the Temporary Internet Files folder if you like, but those files don't usually take up that much space anyway.

Go into your c:/Program Files folder and see if you have leftover folders from any applications you may have uninstalled, but were not deleted automatically by windows.

If you wanna really do the cleaning, go into regedit and delete all those useless registry strings. I don't recommend doing this though if you don't really know what you're doing. But if you wanna give it a try, do a Start - Run, and type in regedit. Go to HKEY_CURRENT_USER and go down SOFTWARE tree. Delete all the folders in there that contains old uninstalled applications. Then go to HKEY_LOCAL_MACHINE and do the same.

Those are what I usually do anyways. Or I would just reformat my hard drive.