Couple of questions: are you experiencing slow network file transfer speeds, only, or an inability to connect, or see computers on your network?
To begin with, make sure you have a good network connection - wired, or wireless on each computer. Then make sure that each computer is in the same workgroup - for XP Home systems this would be named something like MSHOME, on XP Professional systems it would be WORKGROUP. Pick a workgroup name [you can use most any name, like MYHOME, or HOMEOFFICE, etc...].
Next, on your Vista computer(s) go to the Network and Sharing Center and turn on file sharing for the files you want to share with other computers in your home network.
Let me know if these first steps work and if not, let me know what you see and we'll take it from there.
Good Luck,
Lloyd