Today a client friend of mine walked in to my office that his computer kept restarting after he had returned from his trip..after resetting his machine to stop auto restarting..loo…the beknown blue screen of death showed up with this error..showing STOP: c000021a {Fatal System     Error} The Windows Logon Process system process terminated unexpectedly     with a status of 0xc0000139 (0x00000000 0x0000000) The system has been     shutdown.This is error occurs when either Winlogon.exe or     Csrss.exe fails. Usually due to: Mismatched system files have been installed. A Service Pack installation has failed. A backup program that is used to restore a hard disk did     not correctly restore files that may have been in use. An incompatible third-party program has been installed.

Solution. The solution microsoft gives for this is really too technical, thats why my client brought it to me…however, try to boot from the last known best configuration or if you have a bootable xp CD, perform a repair of the operating system at the second setup screen.. That fixed my problem..

http://support.microsoft.com/default.aspx?scid=kb;en-us;156669