Nope, you don't need IIS if you are using FileZilla Server and just need a FTP server. FileZilla does a much better job and is much more secure than IIS for Win2K could ever hope to be.
If the wired and wireless systems are on the same router, there's no reason why they shouldn't be able to see each other- that's really strange. How is your network layed out?
Is it:
Modem >
- Router >
-- Computers (wireless and wired)
or is it:
Modem >
-Computers (hard wired)
-Router >
--Computers (wireless)
If it's setup the second way, and the computers aren't on the same device, that would explain the problem- you may need to set your router/modem to operate in bridged mode. Then you will have to go into your modems control panel and open the ports.