UPnP (Universal Plug and Play – Streaming audio)

This is a technology making streaming AV possible.

The UPnP™ Forum is an industry initiative designed to enable simple and robust connectivity among consumer electronics, intelligent appliances and mobile devices from many different vendors. As a group, we are dedicated to making the connected home and lifestyle mainstream experiences for consumers - and great opportunities for the industry.

Source: http://www.upnp.org/

Of course UPnP has its own nomenclature:

Server: streams the music to UPnP clients in the network,
Renderer: a device with can play the music received from the server
Control Point: the remote control.

XP and Vista are both UPnP compliant.

XP can be used as a server only, Vista can be both a server and a client (renderer/control point).

If you enable media sharing in Vista (an option in Windows Media Player) the PC becomes visible as a media player in the network.

If you are looking for a UPnP server:

check user forums before you buy. A lot of these server software is very buggy and/or cracks under load. Mature products like TwonkyVision don't have these problems.
check if it is DLNA/UPnP certified.

UPnP is incorporated in DLNA.