I tried repeatedly to connect XP and Mac to Vista without any luck. But I have been able to connect Vista to XP and Mac. XP was easy. Vista to XP connected no problem. Vista to Mac took a little more work.
There are some network protocol settings to modify on the Mac. Also, create a user if needed.
The key that was tripping me up was on Vista. You need to modify a registry key in Home Premium. Business and Ultimate provide an interface through secpol.msc.