I want to improve my wireless network

I have a wireless network at home with the following configuration:

There is a wireless router with an old PC with windows connected to it (with an ethernet card). And two portable computers: a PC and a Mac using Wireless connections.

All work properly, but I want to improve my home network with new services.

I want to share all kind of files between all computers. Now I have installed a ftp server in the old pc and an ftp client in the wireless PC. I can share archives then, but I must install an ftp client in the Mac to do the same. but allways between Mac an old Server, not between wireless portables.

Being both windows, old PC (98 SE) and wireless PC (XP) ¿Why Can't I see each other when I explore my network?

¿How can I share the printer connected to the old windows server?
¿How can I share easily, all kinds of files between the three computers?

Thanks in advance,
Carlos