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...