These are just things that could be a problem so don't get your hopes up

Go to Start --> Control Panel --> Administrative tools --> Services

and make sure that the services called 'Plug and Play' and 'Universal Plug and Play Devices' are started. Also have a look at the other services there and see if there is something out of the ordinary.

Now go to Add/Remove Windows Components. Under Networking Services make sure that 'Internet gateway device....' is turned on.

I would really try to get portforwarding to work for Limewire though, instead of using UPnP. I use Media Center Edition as well and have had a lot of trouble with UPnP. I use portforwarding for everything now.

This site has all the info you need to set it up:
www.portforward.com