Your computer shutting down may not be a malware problem.
In Windows XP, the default setting is for the computer to reboot automatically when a fatal error occurs. An alternative is to turn off the automatic reboot feature so you can actually see the error code/STOP Message (which is also known as the Blue Screen Of Death (BSOD)).
To change the recovery settings and Disable Automatic Rebooting, right-click on My Computer and select Properties > Advanced tab. Under "Startup and Recovery", click on the "Settings" button and go to "System failure". Make sure that "Write an event to the system log" is checked and that "Automatically restart" is unchecked. Click "OK" and reboot for the changes to take effect.
This will not cure your problem but instead of crashing and restarting you will get a blue diagnostic screen with information displayed that will allow you to better trace your problem. Next time your computer crashes copy down the entire error message (including all the numbers) and post it back here.
Also, you could run a full system scan with SuperAntiSpyware
in Safe Mode
as a double check for malware.How to start Windows in Safe Mode