Right, problem solved. I reset the CMOS again but left it for ten minutes this time, then connected stuff again and it worked fine! So I put all the cables back where they were meant to go and put the side of the case back on, and guess what? It stopped working again, although this time I got one long beep and two short beeps.
Taking a wild guess, I opened up the case and unplugged the USB headers that I have been having so much trouble with lately (they dont work, or I can't connect them up properly) and sure enough, the computer started! So it's all sorted, just got to go back and re-apply my little overclock
thanks for the help though people, it's nice to know when I have a problem that there is always someone there to help me with it!