Win XP is notorious for peer 2 peer problems, especially if a non XP machines is involved. In essence to be able to use resources on a XP computer you need an ID and login for that computer. If only one computer is providing the resources then you can get by, just create an account for every user on that computer and they should be able to login across the network and access them.
Would then be best to map a drive of the others to that machine and set that as the default storage location.
You can set each machine with its own IP address as either static or DHCP without having trouble with the Internet if you use one of the reserved IP number groups, for hom networks most peole use the 192.168.0.xxxx where xxx is the the machine IP. MS Internet Connection Sharing defaults to the machine with the connection being 192.268.0.1
The full instruction to do what you want could many take pages in order to cover every contingency, bets to do some detail research first or just hire someone with experience.