I have the same problem as...
Asked 2006-11-04 by tik2tak in Windows with 2 replies
Hi everyone! My windows xp logoff immediately after logon. So I have no chance to do anything in windows. This happens in safe mode and for all users too. The system was working properly before it and it was not connected to network for virus infection.

After running virus checker, I had about 7 virusus of type Vi?? In System files including cmd.exe - After virus checker dealt with them, and restarting, I logon and it logs off straight away as described above.

I tried reloading windows in recovery mode, but still have same probem.

If system restore does not work, you would be looking at a repair installation to 'fix' the issue whatever it may be as you can not get into the OS to try and find anything out.

Repair Installation
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.

Perform a Repair Installation (http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/tips/doug92.mspx)