Have you downloaded and installed the LLTD (Link Layer Topology Discovery Responder) on the XP machines? Without that, the Vista machines will not see the XP machines in the View full map link of Network & Sharing Center: http://www.microsoft.com/downloads/d...displaylang=en
Same workgroup for all pcs? Actually sharing something, rather than just file sharing turned on? Same users and passwords entered on all pcs? Network discovery on?
In setting up my home network, I found I had to turn off the firewalls temporarily before trying to see the Vista machine in the XP My Network Places, then turn the firewall back on once it succeeded. See this page: Vista - Networking Problems - Try This ~ The Blade by Ron Schenone, MVP
The following page has a quite good treatment of File and Printer Sharing in Vista, complete with screenshots: File and Printer Sharing in Windows Vista
File and Printer Sharing in Windows Vista
Another good article: How to share files or a folder in Vista
How to share files from a folder on Windows Vista