Inconsistent Networking on XP Pro/Vista Machines

I have a new Vista Business laptop that I am networking to my existing XP Pro network (5 machines). The Vista immediately recognized 3 of my XP machines, and they returned the favor, but 2 of the machines seem to be invisible to the Vista machine, although they're fine with the others.

When this sort of thing happened in the past I usually installed various additional protocols and services in the Networking menu (TCP/IP etc) until everything worked, but Vista doesnt seem to have anything more to install.

Any suggestions would be much appreciates,and I could provide more details if that would be helpful. Part of it may have to do with my newbie-ness on Vista.

Would the fact that I am connecting to the network wirelessly (while the exisiting machines use Ethernet) make any difference?

thanks!