I think I have a working solution for all these Vista to XP connection
problems. It worked for me, it should work for you:

Step 1: If you are using one of these "Internet Security" suites, go to the
firewall section and create a rule that authorises your computer(s) to go
through the firewall (in and out, on the TCP/IP protocol, UDP is not
absolutely necessary). You just create an exception and you specify the
computers by their IP address (individually or with a range of IP addresses).
You do this on each computer and, in each instance, you authorise all the
other computers you wish to "see your computer and be seen by your computer".

Step 2: for those "Internet Security" suites that offer a "map your network"
feature, go to the "mapping page" (accessed from the welcome page, in
general). Then for all computers that are not the one you are working from,
indicate they are "fully trusted" rather than "protected" (the default
option). Do this for all the computers of your network.

Step 3: Make shure that all the drives, folders and files you want to see on
the network from other computers than the one they reside on are shared with
the appropriate permissons. As a starter, allow "all users" to "read" these
drives, folders and files chosen. Do this for all the computers of your
network.

Step 4: Follow all the other recommendations you read about:

a) enabling NetBIOS over TCP/IP
b) making sure your Vista netwok is "private"
c) setting Vista network discovery to "on"
d) setting Vista file sharing to "on"

Step 5: Now that it works and that you see all drives, folders and files
desired, modify permissions (e.g: allow writing, modifying or even full
control) and access rights (less than "all users") as needed.

Good luck to all of you!

--
You need not hope to undertake nor do you need to succeed to persevere
(William of Orange-Nassau)