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
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.