Just installed Vista Home Premium x64 on a new build, and DP worked fine without SP1, and it works fine with SP1...
*edit - wouldn't know why, but could it be with similar AV's... I know having multiple ones can cause error's over the internet, which I've ran into before...
I'm using Avast... but I can't remember what to type in RUN to see all the socket stuff... :/ something with win in it? win______2