Networking between a XP and Vista computer is tricky. First of all, the default names Windows gives to your computer doesn't match so you'll have to change the name of the network on one of the computers. I know XP's default name is XPHome. I forgot what Vista's in.
Since I'm no expert in this, go to Windows Mail in Vista. Click on Microsoft Communities and go to microsoft.public.windows.vista.networking. They'll be able to help you.