File sharing - Windows & Leopard - router?

Like many, I am having problems on my home LAN getting Windows PCs to show up automatically in Finder/Shared. I've scoured these boards & many others looking for the solution but I'm still frustrated.

My current thinking is that this must somehow be related to my router. Maybe, maybe not. But here's what I do know...

Home LAN config:
3 Windows Vista PCs
1 Windows XP Pro PC
1 MacBook Pro
DSL modem (Speedstream 4200) in pass-through mode to...
D-Link router (DVG-5102S). It's a Telephone Adapter for AT&T Callvantage VoIP (but it's a router). The D-Link Router connects into...
Netgear 8-port switch (not a router). This feeds 2 of the 4 PCs (wired). It also feeds...
Airport Express. In bridge mode. This feeds the MBP and 2 of the other PCs
File sharing configured on all PCs & Mac
Firewalls off on PCs and Mac ("accept all incoming connections" on the MBP and Windows firewall turned off on the PCs)
No other software firewalls on any of the computers
Router & AE are on the same subnet

Home LAN scenario:
All 4 of the PCs can see each other and can see the MBP when I click on "Network" in Windows. No problems
All 4 PCs can exchange files amongst themselves as well as with the Mac all according to the specific folder access rules on the PCs and Mac. No problems.
I can connect the MBP to any of the PCs by doing Finder/cmd-K and entering smb//<pcname>. I do not have to use the IP (although that works; too)
For what it's worth, I have iTunes on all machines (PCs & Mac) with iTunes sharing turned on. All of the machines show up to each other in iTunes automatically and can share with no issues

What's not working:
None of the PCs show up automatically under Finder/Shared. Obviously, if I do cmd-K smb//<pc name> it appears there with the "eject" button next to it but as soon as I reboot, it disappears again.
I also use the MBP at work. The scenario there is brighter:
Shared PCs on the office LAN all automatically appear in Finder/Shared
Server drives automatically appear in Finder/Shared
All machines can read/write/share according to sharing rules set up on each machine. In other words, the office LAN works as it should
I've been puling my hair out trying to figure out the missing link (pun intended) to get things to work fully at home. Granted, I am in better shape than many with the "no Windows shared computers" on these boards - I can at least access the Windows machines 100% of the time using cmd-K smb/<pc name>. But I shouldn't have to; something's not right.
I'm not a LAN expert by any stretch. But given what I knew before & what I've learned in the last few days trying to resolve this, it seems like whatever the PCs are broadcasting across the LAN to say "hey, I'm here and I have shared folders" is being blocked before it can get to the MacBook Pro. That broadcast is clearly getting to the other PCs because they all show up under Windows. And the broadcast from the MBP is working, too because it shows up to all of the PCs.

I guess it could be the MBP "blocking" or ignoring it but I don't think so. When I take the MBP into the office, all of the PCs and servers automatically show up in Finder/Shared.

So my "smoking gun" is the D-Link router. I guess it could be something in the DSL or AE but both are in bridge mode so I doubt it.

Help???