Is performance lower on Windows 7 x64?

So I have a custom built desktop. It was actually out of commission for awhile because of bad power supply. I recently got it running again, installed Windows 7 x64, and purchased a few new games. I feel like the framerates I am getting are lower than what they should be. The specs are:

Motherboard: Asus m2n sli-deluxe
CPU: AMD AM2 dual 4200 (2.21 GHz)
RAM: 6 GB ddr 800
Video: Nvidia 7950 GX2 1 GB PCI E
Hard Drives: They are older than the comp is, 7200 RPM I believe.

I turn everything down to minimum on team fortress and still get framerates in the low 20's in lots of action. I was wondering what my bottleneck might be. I am curious if Windows 7 might cause a lower performance because of being new (optimized drivers?, etc?). Also I wonder if the virtualization of the 32 bit games is causing any lower performance? Any insight would be greatly appreciated.