Make sure any firewalls are set to the lowest security setting and then disabled.. Some of them will continue to block ports after you disable them so doing that alone will not suffice.
I have a LOT of experience with computers, but getting Windows boxes to talk to each other reliably has been a very rare experience for me. Often after several hours of messing around I'll get everything to work until one of them is rebooted or I just wait a few hours, then it craps out.
The only way I've found to reliably network computers is to run Linux on all of them, but that's probably not the answer you wanted to hear.
