Windows 7: Computer Crash

Hey,
A week ago my computer started to crash in a sudden.
It suddenly turned off, the power of the computer turned off except the modem etc... so it can not be an electricity crash.
What can make the computer just to turn off suddenly? probably something inside the computer, manually.
Edit: No viruses on my computer... My anti-virus doesn't report about any virus.

It is possible you had a power fluctuation from the power company. Some devices, like your modem, can be unplugged for a second or two and then plugged back in, and they are fine. Others, like computers, respond to voltage drops by shutting down.

I wouldn't worry about it unless it becomes a problem.