There are many articles about how Vista has completely changed network sharing that makes it difficult to say the least to get it working reliably.
There is a patch I believe for XP that helps, a quick google should find it for you.
Microsoft also have an article here
File and Printer Sharing in Windows Vista
TBH I find it amazing that a company that 'sells' its OS on the 'it just works' train of thought should change something so fundamental and then offer no wizard/help within the OS to get you up and running is pure icing on the cake.
I'd offer more help but TBH the more I see of Vista the more I REALLY don't want to get involved.
Perhaps I'm just getting older but I really can't be bothered to invest so much time that Vista seems to need to understand all its bugs. I'll concentrate on my first love more as I believe a lot of you will be joining me before long.