Random shutdowns?

Hello,

I build this computer a few years back, and for the most part is has ran completely fine. Very recently it started to randomly shutdown. Most of these shutdowns occur while playing a game. The last few times I've had this happen, when i booted back up there was an error: floppy disk fail (40)..I don't have a floppy drive.

AND, the first boot menu screen displays the master and slave. The master was read fine and normal, but the slave wasn't even being recognized. It said next to slave: none.

E4500
8600GT
WD 40GB HDD
EVGA 680i
PSU came with the case - so an el cheapo
2GB of Corsair PC-5300

Yes the slave is the dvd

Only mouse and keyboard are using USB slots

My friend has an extra PSU; I can see if I can borrow it

Edit: Theories -

PSU is bad

A virus? That has nothing to do with before the system is booted into windows, though.

Overheating and shutting down. How can I tell if that's the reason it's shutting down? Is there something in windows that documents shutdowns?

I don't really know what else it could be or how to eliminate the possibilities.

Edit 2:

A MSCE at a friend's work said to run a memory test. I did run Windows Memory Test, and I found this quite interesting. There's 6 tests in this diagnostic, and the machine shutdown every time I ran this test. Never did I get to the 6th test, I always shutdown on test 1-5.