I've got a home network with three XP machines and one Vista machine that works fine.
If the IP addresses of all the machines are in the same subnet (i.e. 192.168.100.1, 192.168.100.2, 192.168.100.3, etc) then it is not an IP problem, but probably a workgroup problem.
Just run the network setup wizard on both machines and make sure you type the same workgroup name on each machine.
If you go to Network Neighborhood (XP) or Start > Network in Vista, you should be able to browse the "Entire Network", double click on the workgroup name, and see all of the computers that are currently on the network.
As long as the machines are connected to the same router, that router is assigning IP addresses to each machine, and the computers are in the same workgroup, that should be all you need.