Your computer may be overheating, in which case it shuts down for its
own protection.

Does the computer become louder before it shuts off? That could be a
sign that the cooling fans are working harder to cool the computer.

Something to do: Open the computer case and /carefully/ blow out the
dust and accumulated dirt. That may help.
---
Leonard Grey
Errare humanum est

Garvit wrote:
Since Yesterday (22/Oct/2009), my PC is automatically shutting down after 1 -
2 hours of use. I scanned my PC for any virus using Airtel PC Secure, but
there was no Virus(es). Is it a System Failure? Can System Restore Help?