XP and Vista on a network was always going to be difficult.
The first thing to check is that the Network name is identical on both machines. XP and Vista have different default names so one needs to be changed.
Next hurdle is to ensure that the user logon name is also identical on both machines OR place all shared documants into the relevant shared areas on each PC "All Users" on XP for example.
It's been a few years since I did mine but I got there in the end.