I cannot see some computers on home network

[mozoneman]

I have a 5-computers home network, 4 desktops and 1 laptop. They all run XP, except one with Vista. In addition, there are printers attached locally to 2 desktops through USB. One connected thru ethernet, the other four use wireless. The diagram is as follows (they all have same workgroup):

Computer 1: Desktop, Vista, ethernet
Computer 2: Desktop, XP, Wireless, Brother Printer, Canon 9900 Printer
Computer 3: Desktop XP, Wireless
Computer 4: Desktop XP, Wireless, Canon 9100 PRinter
Computer 5: Laptop, MacBook Pro, OS X 10.5.6, XP emulation thru VMWare, Wireless

There are 2 perplexing problems that keep nagging me for several years now.

First, I cannot see some desktops from others. For example, all the computers cannot see Computer 4. I know this is a Mac Forum, and I apologize for posting this first question here. But I would appreciate any help.

Second, Computer 5 in OS X cannot connect to the printers on Computer 2. The VMWare however successfully connects to those 2 printers, so I do have access, thru Windows applications only. It would be nice to connect for the Mac applications also.

I suspect some network configurations were not set up properly on these computers. Does anyone have any suggestion, or a checklist for me to check?

Thank you in advance.

Huan