Computer restart problem

Hello,

This problem has been happening since late October if I remember correctly. Basically, my computer just reboots all of a sudden. The screen goes black and my computer starts up again from the hardware detect screens (the one were you can go into the BIOS). I can only compare the way it reboots (if you can call it that) to unplugging the computer and it just switching off/the power getting cut and then plugging it back in and turning it on again (without the need to actually turn it on, it just cuts out and blackscreens and starts up again). I don't have to be doing anything in particular for the reboot to occur, I could be at desktop, using the internet, gaming etc. It doesn't happen during any particular activity.

I have asked a few people about the occurence, first they told me to turn off the automatic restart thing, which made no difference at all as its not really 'restarting', it's just switching off and booting up again immediately. Which makes me believe it is a hardware problem which I then asked about. They said it could be the memory or the power supply. I ran memtest and it ran through once (one pass) and it encountered no errors. Which narrows it down to the PSU. So basically I'm assuming it's a PSU problem and therefore I need to buy a new one. But basically I'm looking for a 'second opinion' I guess, before I pay for a new PSU I want to be sure it's definitely a PSU problem, so could it be anything else on my system or am I right in buying a new PSU?

Thanks.

My computer spec btw:

Intel Pentium Dual-Core E2180 running @ 3GHz
2GB Corsair XMS2 DDR2/PC-6400 memory
Gigabyte GA-73PVM-S2H micro-ATX motherboard
GeForce GTX 260, 216 core, 896MB
CoolerMaster Real Power 520W modular PSU
2x HDD, 1 Maxtor 160GB, 1 Hitachi 300GB (both SATA drives)
Windows XP 32-bit