xp:start up then shuts when log on screen appears!!!

Hi all!! desperate for some help!!! my friends computer now has a serious problem!!! when i switch it on, it boots up all ok, then when it reaches the log on user logon screen(blue, with what user name/s there is on the pc)...it just switches off/reboots!!!! is there a solution!!! please hurry,....were desperate!!!
thanx

Lee

Hiya oldspeedracing :wave:

Does this thread help in any way?

http://www.techhelpforum.com/showthr...hlight=reboots