Why doesn't Vista see other machines?

I have a small home network that runs through a router. Vista only sees
itself in Windows Explorer network section. If I typed in \\computername in
Windows Explorer, that particular machine will show up under the network
section. However, that same machine doesn't automatically appear in the
network section when I close/reopen Windows Explorer. The other machines on
the network are
- XP SP2 with Link-Layer Topology Discover Responder installed.
- A NAS running NT 4.5.

The Vista machine can ping both of the above fine and access as I mentioned.
I just want them to always show up under the Network section.

Also, when I click Network in Windows Explorer on the Vista machine, it
takes a few seconds for anything to appear. On my XP machine, this happens
almost instantly.

Any suggestions?

Thanks,
Brett