My guess is that Windows is simply unable to use bridging with the 802.11b card you have. Supposedly, there were a number of cards out there that tried to prevent you from changing their MAC address, and transparently passing the MAC address of the original client is probably the most common way to do bridging. Some open source OSes have overcome that, IIRC, but I doubt that Microsoft is going to try to intentionally bypass security features of a piece of hardware, even if it is for a useful purpose.

Of course, this is all conjecture. I didn't even know that Windows had a bridging function.