Windows XP starts but restart automatically during GUi Welcome screen

Hi,

This is a very unusual error. The Windows XP Pro goes past the please wait screen and then goes to the welcome screen which is usually followed by the logon screen with all the users.

When the PC gets to the welcome screen it displays this message (click on this link to see the message: http://www.marcomnet.net/demo/xppro.gif ) and restarts automatically after 2 secs only! it doesn't respect the time remaining of 29 secs, even in safe mode I can't access windows I tries last known good, the same. I tried every options bu nothing works, I've resolve some complicated startup problems in the past but none like this the system won't let me do anything.

Thank You for your help!

Mario