Windows is not a normal topic of discussion here, but I struggled to find any information to help me with this problem, so I thought I'd post my solution.
After running combofix to clean up some viruses, proquota.exe was found to be missing from my system32 folder and combofix kindly reinstalled it. Unfortunatly, proquota.exe limits the profile size of user accounts, meaning that you can only store so much (10 MB in my case) on your profile (which includes your Desktop, MyDocuments folders, etc.). As a med student, I have a LOT of documents that I like to store in MyDocuments, so this was unacceptable.
If you have XP Pro or Server2003 you can easily find instructions to disable proquota; however, if you have XP Home, you're more or less hooped.
To disable proquota I had to make the following change to my registry (remember, hacking your registry is ALWAYS risky if you don't know what you are doing):
First, go to Start->run and type in "regedit".
Then in the Registry Editor, go to HKEY_CURRENT_USER->Software->Microsoft->Windows->CurrentVersion->Policies->System
Finally, double-click on EnableProfileQuota and set it to 0.
Restart your computer, and proquota will not bother you again.