XP Computer showing in Vista Network Map, not Workgroup List

> Had a home network comprised of 2 XP computers connected to a router.
> Swapped out one of the computers with a new Vista computer. When the XP
> computer tries to access the workgroup, it gets this error: "[Workgroup Name]
> is not accessible. You might not have permission to use this network
> resource. Contact the administrator of this server to find out if you have
> access permissions. The network path was not found."
>
> I installed the LLTD update on the XP computer and now the XP computer will
> show up on the Vista computer's Network Map, but it won't show up in the
> Workgroup Computers list. The XP computer gets the same error when trying to
> connect to the workgroup and I cannot ping the other computer.
>
> Both computers have the same workgroup name. I've turned off the Windows
> firewall and Norton firewall--no change (even though these were active and
> the network worked before)
> I've tried both Public and Private network types on the Vista machine, and
> it's an unmanaged network. No username or password required.
>
> Doesn't seem like it should be this hard....Any other ideas?