Assuming you have Network sharing and discovery properly enabled on the Vista system, and all systems have identical "workgroup" names -- it helps if you have identical User Names and Passwords for the systems that want to see each other -- especially where Vista is concerned.
The XP systems should be able to see Vista on the Network, if not access it, in any case. If it is not being "discovered" try this from XP: Start > Search > Computers or People > Computer > Vista's Computer name.
You also can try this on Vista if the attempt to access is getting "denied":
run gpedit.msc > Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options > Accounts :: UNcheck Limit local account use of blank passwords to console login only
You should also try to workout whether the problem is related to wireless settings. Since wireless routers have the ability to connect by wired connection -- if it is practical -- try it.
Othe issues may relate to router related encryption and passwords for wireless systems, known as "WEP".
Edited by Rollin' Rog, 25 October 2007 - 01:43 AM.