Open security centre and Web and email protection firewall and connections and see if ip addy of other Pcs are allowed. I also suggest setting the other PCs to a fixed IP. If you are using a Router you then need to set the Default gateway and defauft DNS address sto the router IP.
Set all Pcs to same workgroup and then see if you can see the PCs on the Home network /My home network tab. If there trust them.
I am running win7 win7 so cannot check the XP win7 network but it should work.
If desperate before above disable the firewall and try connection to see if it actually is the firewall