Using FTP to share files with 3 Windows computers - Not Working Anymore

Question

My office is trying to share files with a number of computers, both Mac and PC. The files are housed on a Mac OS 10 version 10.6, and I've enabled both AFP and FTP sharing. The macs in the office connect just fine, but yesterday, after three weeks of working perfectly, the Windows machines stopped connecting to the Mac. They get a "The Connection with the server was reset" error. We don't use an FTP client - I just had the PCs connect to the Mac by typing ftp://address in their explorer window.
I've tried restarting all computers, disabling and re-enabling FTP sharing, and I've triple checked permissions, etc. Is there anything else I should be investigating? I'm trying to look into our firewall here, but since our computers are rentals and another company controls our internet access I'm not sure what I can do there. (I tried setting up workgroups, but said company won't give me administrative access over the PCs or access to the firewall, and there's no IT dept to speak of)
As I'm sure you can tell, I'm just a regular old office monkey who inherited the "IT" job with no IT experience, so I apologize if any part of this question is stupid.

Answer 1

I know just about nothing about this, but common dianostic practices include looking at the logs.
Either use Applications -> Utilities -> Console, which has a search function that might make this easier.
Or go looking around /var/log (which you can use Unix 'grep' to do your searching ). Then again, NOT ALL logs are in /var/log, and Console knows where more logs are located.
2 logs that might be a good starting point would be /var/log/messages and /var/log/secure