Vista changed it, probably with the entire network rewrite that came in that dropped the tried and true evolved BSD networking code for internal Microsoft crap. One of the reasons Vista was on my do not use list, the network stack just did some stupid things when it was in a mixed network environment. The paid service pack to Vista seems to have helped quite a bit, but it's still not back to where it was in XP.

*edit* Gah, keep forgetting the BSD TCP/IP pieces were only in until NT 3.5 timeframe, back when the network stack was licensed after the OS/2 divorce with IBM. Anyhow, still a lot of issues came in when MS rewrote it for Vista.