You can find it in device manager, under the network card's advanced tab.
I haven't confirmed this will fix it in Vista, but it looks to be a problem with Winpcap not being able to see the packet properly when offloading is used.
On some cards, checksum offloads are split into IP, TCP and UDP, and I believe, it's either TCP or UDP, depending on which is used with GIT. But on most NICs, it should just be one option - all or nothing, so-to-speak.
Cheers!