Do you get approximately 1 minute before it shuts down, if so the infected with sasser worm.
Stop the Shutdown Cycle
This worm may cause LSASS.EXE to stop responding, which forces the operating system to shut down after 60 seconds. If your computer starts to shut down, follow these steps to abort any system shutdown that may be in progress.
1.On the taskbar at the bottom of your screen, click Start, and then click Run.
2.Type: cmd and then click OK.
3.At the command prompt, type: shutdown.exe -a and then press ENTER.
click here for removal instructions.