I don't know how to tell whether it is working or not. The diagnostic LEDs light up when it's on and everything.
Is it possible that it's my hard drive, somehow infected with a Trojan? I'm pretty sure that my PC has grade A security, but who knows.
I will try taking my CPU out tonight, and see if it does anything when I reinsert it.
If not, I'll try taking a look at my HDD. Could be that, by some small chance.