I'm getting the exact same issue.
System hangs completely when browsing the net, using either IE or Firefox, with any AV installed. Other than that, Windows 7 has been stable.
Interestingly my hardware specs are very similar :
Intel(R) Core(TM) 2 CPU 6600 @ 2.40GHz (2 CPUs)
NVIDIA GeForce 7900 GTX
Realtek HD Audio
mobo Abit AB9 Pro
I'm not using a wireless network, though, but a wired one using the motherboard connections. It's not a generic network issue, as I have played online gaming for many hours under Windows 7 without issue.