1st - Get rid of the wired router and just use the wireless...it should have a few wired ports on the back of it.
2nd - get rid of zonealarm and any other firewall software you have. The router is a good enough firewall, and those programs often block computers on the local network from having access to your computer, not just internet h4x0rs.
3rd - make sure you have user accounts on all the computers you want to have access to. For example, if you're logged on as "Admin" on computer A and you want to access files on computer B, computer B must have a user account called "Admin" and the password must be the same too.
And, windows XP/2000 can be finicky if you're sharing the root of the C: drive.
Oh yeah, and obviously they all should be on the same workgroup.
