Thanks for all those who replied here. I've taken out one of my RAM sticks and had a friend test it. It works perfectly in his PC. And with it out of my PC, my problems dissappeared too. The moment I put this RAM stick back it reboots again at startup.

I've had two friends bring their RAM over to test in my machine. One's RAM caused reboots like my RAM does. The other works perfectly.

Any ideas?