Random freezing in Windows games over Steam

In the last three weeks, I've built a new rig. The specifications are as follows:

Gigabyte GA-880GA-UD3H
6gb RAM
XFX Radeon HD 5770
WD 160gb sata
LG DVD ROM sata
AMD Phenom II x4 3.4ghz with stock cooler and not overclocked
Coolmax CUG 950B 950w psu
Windows 7 64bit

The Details of the Problem

While playing MW2, Rome Total War, Medieval 2 Total War, Empire Total War, the game and subsequently the computer will freeze up hard. I cannot launch task manager or alt-tab out of the window. When any of these games freeze, the sound reverts to an engine like sound that will not stop until I do a hard restart. It may be of interest to know that Elder Scrolls Oblivion does not freeze on this computer.

When I first built the computer, I used an Asus motherboard. Upon finding out that the PCI-E ports were DOA I installed this Gigabyte board. Is this problem due to Windows needing to be reinstalled? Or is it something else? I'm clueless at this point!
 
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