When my system with Windows XP Log on and automatically Log Off

what is th problem

When my system with Windows XP Log on and automatically Log Off

what is th problem

it could be a bad power supply or it could be software problem even a hardware, problem like bad ram!,the list goes on and on!
but the most likely cause is virus/trojan related .the fix depends on who you get to look at it for you .
hitting f8 on startup can you get into safe mode to run virus scan

When my system with Windows XP Log on and automatically Log Off

what is th problem

I have seen quiet a few problem like yours.

The solution is reinstall the windows.

Hope this helps!
Bill

I would still try booting into safe mode with networking and then running all of the virus/spyware scans. If you still have internet connection, try Trend Micro House Call. It is a free online virus/spyware scanner. You could also try to download BitDefender Internet Security. There is a trial for 30 days and it should update and work just fine.