PC restart problems..

alrite, i recently bought a new pc consisting of:
E8500 Dual Core (3.16ghz)

2x2 (4GB) dual-channel RAM

ATI HD4850 512mb

and a 500w power supply.

The PC ran fine for the first week or so, now.. for some reason if i just tap the case even a little, or bump it, it restarts.. also, it sometimes randomly restarts when i dont even touch it..

Anyone know whats wrong?

Check if all your cables are connected properly, loose connection can result into auto-restarts/shutdowns.

Whats the brand of PSU did you buy?

The longer way to go about this would be to remove all your hardware to put them back together again, making sure everything is secure and tight in their respective positions. If that still doesn't solve anything then a piece of hardware might be defective.

I'm curious though about your PSU. Is it generic? Does it output a constant 500w or is 500w just its maximum output? What's the amperage on the 12v rail(s)?