Try if Safe Mode with command prompt work to do a system restore: http://pcsupport.about.com/od/fixtheproblem/ht/systemrestorecp.htm
If not, google XP AUTOMATIC LOGOFF or XP IMMEDIATE LOGOUT (or the other 2 variations) to see if somebody found another way except doing a repair install or even a clean install of Windows.
I boot up, select a user. Windows logs on, opens the users front screen, then immediately goes to "saving settings, logging off", and back to the select a user screen.
Same in safe mode.
Csn't do anything to get into windows.