Computer restarted itself repeatedly, after leaving it on all night (presumably restarting itself occasionally) it no longer starts at all. No POST just a little fan spin.

A few nights ago my computer began restarting randomly, doesn't seem to matter what I am doing, mid game or even in the middle of starting up again it would reset.

- I disabled auto-restart on error and I didn't get any blue screen of death and I cant see any errors in the log....so not sure about that one.

- I cant run a full system virus scan (it resets midway through) so I cant see if that's the problem.

- Cant run memtest (it resets midway through) so also cant see if thats the problem.

- Ive cleaned out the case, all the fans are working and the hottest component is the GPU sitting at 60-65C so that shouldn't be the problem.

- I'm fairly sure theres no loose connections but ill reseat everything just incase.

- The amount of fragmentation on my HDs is incredible......its freaking amazing the poor thing works at all.....I haven't done a defrag in ages so I tried running one the other day, I left it during the night assuming if it still stuffed up it would just reset a few times....now it doesn't start at all.

So as far as I can tell PSU is the most likely cause, when I go to start it now there are some lights on the motherboard and I get half a spin out of the fan but that's about it, no POST, however the PSU is one of the newer components in my box, mine kept dying so I forked out for a good quality 750W.....cant believe one has gone again.
GPU is a possibility, its one of the oldest components and its been bugging me for a while, not sure if it would cause whats happening now.
Bad memory as a possibility?

I'm not sure other than that, if you have any ideas please pipe in.

Thanks for any help you can give!

could you post more specs on your system? ie brand and model of the parts.