Sorry to tell you it could pretty much be anything
Had a similar problem on a machine last week - was spyware related but the account would start login in and then crash and reset. What I ended up doing was:
1. Booted up n safe mode and went in as administrator.
2. Created a new account with a similar name
3. Changed the security attributes on the documents folder of the original account to enable the second account to access them.
4. Deleted the original account.
5. Ran adaware and the Microsoft anti-spyware software and installed and ran avast (anti virus)
6. Rebooted in normal mode and logged on under the new account.
It was so badly in/affected that trying to clear out the temporary internet files for the old account causes a crash and reboot.
Probably the easiest way to clear it.