Computer has suddenly started locking up

My computer has started locking up, first i thought it was to do with Oblivion as i was palying yesterday and the screen just froze, so i reset and then restarted the game about 20 mins later it froze again so i reset and restarted and about an hour in it froze again.

But today it has locked up 5 times while i am just browsing the internet, nothing works i cant cntrl, alt, delete, and the mouse cursor doesnt move and there is no activity going on in the hard drive (little light is not flashing)
the only way to get out of it is to reset.

I have run anti virus and spyware scans and they have come up with the normal cookies that you get, it only started happening yesterday and i have not insatlled any new programs or anything, i really dont know whats going on ??

First i thought it may be overheating so i have cleaned all the fans and the cpu heatsink, temps seem fine ?

CPU= 47c
GPU= 46c

I have run memtest and no problems there.

I have run the seagate barracuda hard drive diagnostic test no problems reported.

I have even reformatted and still it has been locking up.

Here are my system specs

Alienware aurora-fx
Abit Av8 socket 939 k8 motherboard
Amd athlon 64 fx55 cpu
Gaiward bliss 7800gs gpu
2 gig ddr pc3200 ram
120gb seagate barracuda hd
Sound blaster audigy 2 zs sound card
480 watt alienware psu

I purchased this rig 3 years ago and the only things i have changed are the graphics card and added another gig of ram but that was over a year ago, and it has been running fine until now, i dont know what else to check ?

Any help will be much appreciated thankyou.