Ok, this is getting to a point of ludicrocity. I have a laptop running XP, I have a new desktop running Windows 7. The network host computer (and one other on the network) are on Vista.
All computers are sharing the internet connection fine. The three non-7 computers can see one another and interact in Explorer without issue, moving files etc.
Explorer on the Windows 7 computer can and access see the two Vistas but not the XP. The network is set on the Windows 7 as a Work network, not a homegroup, as has been suggested in other threads, and those steps have been followed. The network is named WORKGROUP for all computers.
Where it gets frustrating is that pretty much any other program with the capability on the Windows 7 computer CAN see find the other computers and shared folders, and can modify them, as they should be able to. Both Paint Shop Pro 7 and AudioSurf can quite happily access the XP computer's shared folders, and cut/copy to the Windows 7 HD.
The computers clearly are networked together, on the same network. And if these programs can find the XP computer, why can't Explorer? It seems ridiculous that I need to keep opening up unrelated software just to be able to move files from the XP computer to the 7 one, or vice versa.