Hi,
I'm currently using the RC of Windows Server 2008 R2 (I will be going RTM soon though) and have so far had no trouble at all with it. However, whenever I play Colin McRae: DiRT, it lags a lot. This system had no trouble playing it at all on Windows 7 and has fairly decent specs (Q9550, GTX 275, 4GB RAM). I found out that by changing the DIRT.exe's process affinity to something else, the problem goes away. Whats more, if I change the affinity back to all four cores, there is no problem whatsoever. This system has been able to run every other game I've thrown at it so far perfectly.

What could cause such behavior? I have made a script which launches the program, changes the affinity twice within the space of a second, which works perfectly, but I'm wondering whether theres another problem causing it to happen in the first place.

Maybe try going into Control Panel -> System -> Advanced System Settings -> Performance settings -> Advanced -> Processor Scheduling -> Adjust for best performance of [x] Programs.