Computer Randomly Turning Off

Recently my computer has been having problems while running World of Warcraft. Everything else is running perfectly fine, as well as multi-tasking with programs such as LimeWire, Windows Media Player, AOL Instant Messenger, and so on..

In a matter of minutes, the monitor image gets very shakey and the computer just turns off on me.

My Computer Specs:

AMD Athlon 64 [3000+]
2.01 GHz, 1 GB of RAM
Windows XP Home Edition (SP2)
Envision 17'' LCD
420W PSU
150GB Hard Drive

I've had this computer for about 1 year, and this problem started happening about 2 weeks ago. Please help as soon as possible, thanks.

- Loren

EDIT: Just realising, the computer stays "on" but the monitor shuts off and there's no getting it back on without restarting the computer.

 

What graphic card do you have?