If one day, you are unable to logged in to your Windows because it's just keep auto logoff right after you have logged in with your password successfully. And you have make it back to normal even

  1. Scan whole your computer by anti-virus tools
  2. Repair your Windows with the CD
  3. Anything you can...

I have deal with this problem on my co-worker computer, and found this http://support.microsoft.com/default.aspx?scid=kb;en-us;892893, but still have problems that I will not know which file is declared in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon until I can read your my registry, but how ? Will booting to Recovery Console with the Windows CD will help ?

Finally, I take the HDD out the PC, plug into working one and using this amazing tool MiTec Windows Registry File Viewer to read get thing done, it able to read your offline registry file, in this case you should open system in %SYSTEMROOT%/Windows/System32/Config, then you can follow instructions in http://support.microsoft.com/default.aspx?scid=kb;en-us;892893

Hope it help