HELP! XP <-> Vista networking problem

One Vista computer hardwired to router. One XP fully patched computer
wirelessly connected to router.

Trying to network them with the basics...printer (connected to Vista
machine) and selected folder sharing.

Done all the obvious:

1) Ran the XP network wizard
2) Workgroups are the same
3) Vista settings appear correct

Had this problem a few months ago and this esteemed group recommended
Network Magic. Seemed silly that I couldn't do this within the OS but
gave up and tried NM. For the most part it worked with no effort.
Simply installed on both computers and it worked for the most part.
Some file sharing was goofy, but the printer worked so that was
sufficient. After the free trial was over no change so I didn't pay
for it.

Fast forward to this weekend. Old computer moved out, dug out my old
one for number 2 son. Rebuilt XP from scratch on it, got the wireless
to work and tried to network them. Nothing. Loaded NM on the new
machine...no help. Paid for the software...no help. Called
NM...they're stumped.

Vista computer can see the XP computer on the network, but can't
access it ("Check spelling, etc..."). The XP computer can't even find
the Vista computer on the network.

Ideas? Son desperately needs access to my printer for school work.