Your OS should not be an issue for most PC games. I run W7 64 bit and so far, I've not found a problem running any games on it. That includes really old games such as Tomb Raider 1 which came out in 1996.
Although I love Windows 7 it has thrown issues up for me particularly because mine is X64. Fifa 10 is a no go area due to lag when I plug in my controller, Battlefield Bad Company 2 has a slight but perceptable lag feature which particularly endearing when you find you are shooting at someone when you are already dead! However I don't wntirely blame Windows 7 as both of these have a common denominator........EA! Thats right in this world of increasing ram values in computers and 64 bit computing they do not support X64! quite amazing really for a company that size. For that reason I have had to put an XP partition on my computer. Still the best OS for gaming for my circumstances