Pc shut down when executing spybot s&d

[brainstuffch]

Hi,
I have a notebook with Win xp prof. Some times the notebook try to close all windows and shut down .. only open programs can avoid this and also not in all momenets. I have tried to seek for a Virus and with Spybot S&D for Spyware .. Spybot starts and after some seconds of the scan the notebook shut down. After shuting down the system can reboot normally or some times I have a screen related to the network and it shut down again. I restar the computer and he works normally for some hours.
In the security logs I have fund autortty NT\system and user32

What can I do to finish completely a scan of spybot ?

[129260]

safe mode should be able to allow spybot to do a full scan, and will be able to find harder infections. hit f8 when u see the windows loading during startup, and choose safe mode. then run u scans.