These are just a few thoughts to get the ball rolling ...
What happens if you disable Norton Firewall? If problem goes away, investigate Norton. If it doesn't, probably not relevant.
Install LLTD? Worthwhile for its own sake, but not relevant to your problem. I have a network of XP, NT4 and 98SE machines that works fine; they don't show up on Vista's network map in the proper place, that's all.
More thoughts ...
Is the Vista Basic machine in the same workgroup?
BTW, what service packs are on the XP machines? You need SP2 to be able to install LLTD, but it's included in SP3. However, even if you have LLTD installed, some network adaptors can't use it
It's that man again ...
Don't think it's just Home Basic that has problems networking, from my researches when trying to troubleshoot my own initial problems it seems to be pretty universal across versions.
I'm running Home Premium, so possibly not best placed to help you with this, but I don't believe there's any significant difference between the two as far as the basic networking setup is conncerned.
But, Vista is quite bizarre as far as displaying what's on the network is concerned. Tonight is typical; there are four network devices online, but if I open 'Network' from the Start Menu, only two (wired connection) are displayed - an NT4 PC and a NAS device (in both details, and folders, panes) - this (wired conection) PC doesn't appear! If I refresh the view, this PC appears (both panes). If I type (wireless connection) "\\missingXPPCusername" in the address bar, it appears in the folder pane, with shared folders in the details pane, but never, repeat never appears as a PC in the details pane.
Anyway, back to your situation. This is probably a stupid question, but, like the workgroup name, I take it you do have exactly the same settings for both Vista machines in 'Network and Sharing Centre'?
What happens if you type "\\XPPCusername" into the address bar? Can you 'ping' the XP PCs? What happens if you type "net use somedriveletter: \\XPPCusername\sharedfolder into a command prompt?