I'm not sure that I understand your issue, but if you mean that you cannot turn on the PC and log into Windows, that is to remain logged into Windows to use the PC, and it logs off and shuts down, it may indicate a malware infection.
If it is crashing and powering off shortly after startup and login, then it can be a Windows problem, hardware failure, or any number of other possibilities.
If you are using Windows Live OneCare and you have been infected, but OneCare did not detect or cannot remove the malware, please contact support to report this and for help with removal.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator
