Pc starts- few seconds-no display/beep - off

Hi,

Ok bad title I know, here is the story:

I was intending to put in a new hard drive but realised I didn't have enough SATA cables ( I didn't unplug anything) but in opening my pc I realised that it was quite dusty, so I cleaned it. In doing so ( I assume this is where the problem arrised) I pulled off my heatsink and fan. The first time i put them back on I did it wrong then the second time I was so focussed on getting it on right I didn't plug in the fan.

Now the pc starts with no display or beeps (i'm not sure if I have an on-board speaker if I don't how do I jam 1 on?) then after ~3-10 seconds it just turns off, then a few seconds later it turns back on again and repeats until it turn it off

Any ideas? I'm about 2 go try running it without ram then hard disk etc

Thanks really apriciated (I'm dying here)

Adam

btw running

vista ulti 64bit
gigabyte S series m board
geforce 9800 gx2
ddr 2 ram
1 hard disk (currently)