I have Vista 32-bit on an Intel x86 NVIDIA nForce4 motherboard. Ever since I installed Vista, networking has been... dicey. Any time the computer went to sleep/hibernate, networking would no longer work. Some BSODs on wake. Installing the KB931671 Windows Update reduced the number of BSODs, but not the problem itself.
Updating to the latest nForce4 drivers (15.00, released Feb 5 2007) caused networking to simply stop working from startup. No ethernet, no internet, no connection to other workgroup computers, no ping to gateway even (transmit failed, error code 1231).
Through a process of trial and error, I found that disabling Virtual Machine Networking Services on the nForce Networking Controller, plus netsh interface ipv6 reset from the command line, restored networking. (Virtual Machine Networking Services are installed by Virtual PC 2007 as part of its Virtual Machine Additions.)
So I'm left with no network connectivity on my virtual machines. Perhaps I should count myself lucky I have connectivity on the host, but instead I think, no, a steaming pile of curse words followed by a new NIC will be the answer.
UPDATE June 8, 2007 - Bought a new network card and disabled nVidia networking. All problems resolved. Cost? $35.