XP machines seen from some Win7 machines but not others?

I have a perplexing situation of having 3 Win7 pcs and 2 XP pcs, but the XP machines cannot see any of the network, while only one Win7 pc can see the XP machines!!!

PC 1: New Win7 64 bit: Can see Win7 pcs on network tab of explorer, but can see all pcs if i go to map network. PC 2 is on the network mapo as discovered but cant be place don map.
PC 2: Ex-Vista pc clean install Win7: Can see all pcs both in explorer and network map. Oddly it has a question mark on the netowrk map as to how it is connected ot the router.
PC 3: Ex-Vista pc clean install Win7: Can see Win7 pcs on network tab of explorer, but can see all pcs if i go to map network. PC 2 is on the network mapo as discovered but cant be place don map.
XP PCs: Access denied to ASPLINS workgroup that every comuter is in

All 3 Win 7 pcs have identical network settings on Home network with Workgroup turned off. so there is something odd about PC 2 that means it cant be placed on the network map properly, but that allows it to see the XP machines. Is this somethnig to do with IPv4 vs 6 and if so where do I go to align them all.

Thanks for any advice

Mike

forgot to add I can ping all machines form each other