|Do a Disk Clean Up.|
Clean out your browser's History, cookies, temp files, etc.
Clean out your recycle bin.
Uninstall programs that you never use, especially if it was included with your system and you have no use for it.
Reduce the space that system restore uses from the default 15% of the disk size to 10% or 12%. This just reduces the amount of restore points that your system saves.
Get an external hard drive and off load files that you rarely use, but make a back up first for security.
Check how much virtual memory (paging file) your system is set for, it should be around 1 1/2 times your actual RAM, though only 1 time (=) is fine if you have over 1GB for XP or over 3GB W7 32bit.
You have to be a little bit crazy to keep you from going insane.