The Windows 7 machine is the most likly culprit. Try
Start > Run > CMD > type "netstat -afno"
Right Click on top of window > Edit > Select All > Hit enter key
Start > Run > Notepad
CTRL+V
CTRL+F and search for "111"
This will show if if that machine is listening on, where that port is routed to, and what PID is listening.
Once you find the PID, go to your Task Manager and View > Select Columns and Select PID. Then sort Processes Tab by PID and found the one that was listening on port 111.
If its the RPC, then there you go. If its something else, like a svchost.exe, that gets more complicated, but I can give you commands to find out what .dll's are in that svchost. If you don't see it at all in the Netstat, then its probably something else in your network and I would suggest just turning devices off one by one and doing the scan. Last, if you see the port in netstat, but the PID isn't showing in Task Manager, you might be infected by some kind of trojan using RPC to communicate.