Oh. I know what's happening now. The initial problem I had yesterday was a result of the two machines being on different subnets and ALSO utilitizing different routers for their primary gateways. We have an Adtran that is currently the production router and the Junipers are in limited use for testing and will take the place of the Adtran once we are assured in their operation. One of my test machines has an IP from both subnets and so I think it was trying to send traffic back to the initiating machine by using the IP with the same subnet as the machine sending the request, but that computer was just dropping that traffic because it wasn't expected from that IP. Once I removed the 192.168.42.x subnet from my test machine, the file transferred normally because both of those machines are using the Junipers are their primary gateway. Your response made me realize what was occuring. Well done.
I'm not quite sure why Windows will respond to a request from a particular IP on another subnet by using a NIC on that same subnet instead of just responding on the same IP that received the request. I guess it has something to do with OSPF, but it doesn't work in this case. Granted, it was still my fault.
Thank you.