Interesting - for a non-computer-literate person to have enabled profile quotas!
It certainly isn't enabled by default in XP.
Description: proquota.exe is located in the folder C:\Windows\System32. Known file sizes on Windows XP are 50,176 bytes (50% of all occurrence), 45,056 bytes, 45,568 bytes.
Important: Some malware camouflage themselves as proquota.exe, particularly if they are located in c:\windows or c:\windows\system32 folder.
I would definitely suspect foul play, so a trash and burn followed by a clean install sounds like the best way forward.
SD