Try this out..
Network Map in Windows Vista does not display computers that are running Windows XP

Problem occurs if the Link-Layer Topology Discovery (LLTD) Responder component is not installed on the Windows XP-based computer.
(Install LLTD Patch on XP computer)
Download LLTD Patch from this link:

http://www.microsoft.com/downloads/d...displaylang=en

once u install the patch, XP and Vista shud start working fine.
Do no forget to enable Network Discovery, File Sharing, Printer Sharing, Password protected sharing (this will enable u to have a password when u connect to other comp).. from the Network and Sharing center in Vista..

Hope this helps