If the RISC OS PC cannot access Windows 7 with the Windows Firewall disabled, I cannot see how enabling particular ports is going to let you get connected when the Firewall is enabled.
Try this. Navigate in the Win 7 registry to:
...see if there is an entry for LmCompatibilityLevel and, if there is and its value is "3", change it to "1".
The effect of this is to change the authentication level from "Sending NTLMv2 response only" to "Send LM and NTLM". This fix was sometimes necessary to overcome LAN authentication issues in Vista. I'm not sure if the same applies in "7".