Computer freeze

[mysteryman]

The problem is that my PC keeps freezing randomly maybe good for a half hour or even a few hours but then the screen freezes and the mouse pointer and nothing will move. I've run Memtest for around 4 hours but it doesn't have any errors and I tested the CPU with Prime95 for 3 hours but comes back with no errors. Therefore I don't think its the RAM or CPU. Only ever overclocked my PC once about a year ago but reset everything back to default ages ago. My PC specs are:

Intel Core 2 E6600 2.4GHz
2GB Corsair DDR2 XMS2-6400C4 RAM
Gigabyte GA-965P-DS4 motherboard
ATI Radeon 512MB X1900 graphics card
Tagan TG530-U15 530W ATX2.01 PSU
2x 320GB Seagate Barracuda 7200 SATA-II 16MB cache

Does anyone know what else could be wrong with it? Do you think taking the whole thing apart and putting back together will solve it? Thanks

 

[mysteryman]

Windows Vista. Previously I installed Windows 7 Beta but it kept crashing and giving me a blue screen with the error IRQL Not Less or Equal. So I reverted back to Vista and now the screen just keeps freezing forcing me to restart. It seems as if its ever since I changed operating system thinking about it. Although it was getting slow with Vista previously. Hence all the reformatting and reinstalling.