computer will freeze

I have the following recently built computer.
MSI P6N SLI Platinum motherboard
PCI Express GeForce 8800 GTS
PC Power and Cooling Silencer 610 power supply
2 X 1GB Kingston Valueram DDR2 800
Samsung Spinpoint T series HD501LJ SATA hard drive
intel core 2 duo E6850 cpu

I keep getting random freezes. The computer will freeze or windows will not minimize/close and if I mouse over the task bar I will see the hourglass and the computer will freeze about 30 seconds later. The computer freezes much more when I am away from the computer (for like 10-15 minutes) and nothing is running. If I am running stuff (like a game) it is less likely to freeze.

I have swapped out the video card for a older one and it still freezes.
I have tried both sticks of ram as single but it still freezes.
I have tried different monitor and it still freezes.

I also have a Samsung 18XDVD+-R but the system froze when this was not even connected so i think it is safe. I have had occassions when it was hard to read from this drive but maybe it's because of the other issue with the computer that to this point is unknown.

I have heard a noise like a tick sometimes (not often but maybe once or twice a day) come from the computer and I am not sure where it is coming from. Often times a tock sound will follow sometime shortly after. Once it froze about 1 minute after the tock sound but other times it did not freeze after the tock sound.

I think I also got a message once that the windows sys files were corrupted and I had to reinstall windows XP. It did not crash/freeze during the XP install. One time it froze at the BIOS screen but that was after I was rebooting it from another freeze.

Not sure what the next step should be. If I had a spare PSU and CPU, I would swap them out but I don't have spare parts this powerful. Help is appreciated. Thanks.