Windows 7: Cannot see Windows XP computer

Here's the situation: I am connected to a router (Ethernet) and am the wireless access point for two other computers. Both can access the internet fine. The computer below me can see all connected computers in the network. However I can't see one of the computers (the one on Windows XP) and they can't see me. As a result, I can't set up wireless printing for my room.

Where do I start? I have tried comparing services of the good computer to mine, I changed the wireless network adapter from third party to Windows on the XP computer, I installed the Link Layer Topology hotfix for SP3, and I have even tried changing the network name a couple times too. I even ran the Windows XP Network Setup wizard. ZERO SOLUTIONS SO FAR. Now I will ask the obvious - does this have to do with the bit version of the computer? The affected computers differ (XP 32bit vs 7 64 - and the only good computer that can see all 3 is on 7 32bit).

UPDATE 1: Ok, while I can see one other computer, I can't connect to it at all. Forgot the error, but it says that cannot find the host. And then I tried it on the other host back to mine and same thing there. So it is MY COMPUTER that is the one most screwed up.

UPDATE 2: I did some more experimenting - turned my firewall (Agnitum Outpost) to Allow All mode and then I could connect via the working computer to mine and vice versa. But that doesn't explain how they showed up in each other's network section - maybe this was just relayed info via the Link Layer Topology or something related?

UPDATE 3: I might have it working. I did some exploring of my firewall some more and somehow "Detect new newworks automatically" got disabled. Turned that back on, clicked Detect, and now I can see the Windows XP computer. With problems like these, you can only be out of the woods for a little while. Not totally solved yet.

Ok, so I think I solved it. Seems "Detect new networks" somehow got unchecked in Agntium Outpost Firewall. That is all set, now to get this OLD printer working wireless. No luck yet but that's another topic.