Assuming that you mean the Windows Live OneCare firewall, open OneCare, click on change settings, Firewall tab, advanced, Ports and Protocols. THen add the UDP port there. I don't recall the specific entries, but I believe that there are two boxes for range, put 427 in both. Select UDP as the type. I don't remember the choices for scope, but it will be whatever you choose for both inbound and outbound.
If you have problems with this, please contact support.