Private Network Malfunctions with XP and Vista computers

I have struggled for several days to integrate a new Vista Home desktop into my private wireless home network that contains two older XP computers. I have set up my administrative accounts on all three computers: my username is GAI and my workgroup name is MSHOME. My password is the same on all three computers (is2660).

One of the XP computers (\\MSHOME\hpa450n) does NOT show the Vista computer (\\MSHOME\s3100n) as part of the MSHOME workgroup (I have renamed the Vista's workgroup name to match the older XP convention). The other XP computer (\\MSHOME\dv1240us) shows the whole network and works fine. From this functional computer, I can access the shared folder (__primary) on the Vista computer, revise files, save them back into the Vista shared folder, or print them on the network (see below).

What kind of switches to I have to reset (on either or both computers) to get the malfunctioning XP computer to see the Vista.

Can anyone offer me a prospective solution? I am not a network geek, but it seems that this should be a slam dunk proposition to set up such a simple network in less than 15 or 20 minutes. I'm not beginning to love Vista yet.