Wouldn't you just need another NetGear MP101 receiver for your other sound system?
In this setup, your PC is the server. So you don't need a separate "UPnP server".
*Edit*: Okay, I've been to NetGear's website and see the MP101. You wouldn't need another one of those, necessarily. You would need a box similar to that, but one that just receives the signal from the network, not one that allows you to choose songs like the MP101.
I'll do some hunting...