Not sure where to post this but here goes:
I wonder if anyone can help, I seem to be having problems with my desktop again.
I was just transfering files to a memory stick, and the computer suddenly turned off back on and back off in loop. It wouldnt load, and wouldnt switch off when the button was pressed, so I turned it off at the mains then back on. I then try switching the computer on and it wouldnt fire up. Unplugged it and looked at the fuse, luckily enough we have a spare cable with the same fuse so I replaced it. Plugged it back in, pressed the power button, and the computer came back to life. It was working properly again, so I tried doing what I last tried doing and did the same thing again after a while. It keeps looping on and off, but this time shows a screen asking me what mode I want to start it in (Start Normally, Last Known Good and Safe Mode) I tried all 3 of these but no luck. As soon as I select one it reboots again. What is going on please? I have an incling its the PSU perhaps?

I was last on it on Monday and it was working fine, I've not installed or removed any hardware or software, apart from installing a game which I've had for ages and never caused any trouble, just seemed to happen out the blue

This loop can happen due to many things: Bad sector on the hard drive, memory gone bad, problems with the motherboard. In this situation I diagnose each component with Hiren's Boot CD. You can use memtest86 for the memory and there are many hard drive utilities as well. If everything seems to be in good shape, try repairing your copy of Windows. If you still experience problems then try diagnosing the power supply. I do this with a multimeter and a power supply tester. You can also catch a glimpse of your voltages to make sure they are good in the BIOS, but the PSU may still be bad.

Also try an UBUNTU live CD and see how that goes.