Windows XP - three user accounts, one keeps reverting to default settings.

What the title says. My mom's user account has reset itself twice now, over the period of three days. We still have all the files in the C: drive, but on the account itself, the desktop, itunes settings, etcetera are erased. We kept having to redo everything.

What could cause this?

Insightful answers all get a thanks... If that'll help you to help me.

I worked on the problem all night and figured it out. An important file in the Documents and Settings folder (C: drive) was deleted, so the account kept resetting. Please lock.