XPs default grouping is MSHOME, and I think Pro is WORKGROUP, with Win7 I think it's something else, like HomeGroup, but it should still be able to access the shared printers. Check your network settings to make sure that file and print sharing is enabled, but it should still work without this. Maybe check to see if its on the same subnet (ie each of the pcs and printers ip addresses are 192.168.1.*** and their subnet mask is 255.255.255.0 and they're all pointing to the same gateway and dns (from what I gather with a 192.1.1 address this should be 192.168.1.1 for both)
if the Win 7 is HomeGroup(default) and the rest of the system is WorkGroup, he's not going to share any files, if whatever the printer on, is configured for WorkGroup then it won't see that either, if the priter connection is shared thru a computer then he can't access it either
I've got 3 pcs at home on their default network domains(MSHOME HomeGroup and Workgroup) and the win7 pc can see the shared files on each regardless of the domain, that said the xp home on MSHOME cannot see the homegroup or workgroup domain files unless I navigate to it in explorer. If the printer itself is a network printer it should work, if it's attached to one of the other PCs the win 7 pc should find it no problem regardless of which domain it's on as 7 automatically checks the old default domains.