Make sure that the Windows 7 PC is in the same Workgroup as the other computers. I personally prefer to use a name of my own, but if the rest of your network uses MSHOME, then use that in Windows 7 too.
The network type in the "Network & Sharing Center" should be set as a Home Network.
If you have set any folders in Windows 7 as "shared" then that computer should be visible in "My Network Places" in the XP PCs. Don't get involved in "Homegroups", because that only applies to networking with other Windows 7 computers.