Well, the 'right way' to do this is to set up a Media Server and use that as your source on the home network.
Failing that, another way to do this (and this is loads more complicated than simply running Media Server) is to keep your music files on a shared network drive or nas (naslite.com, openfiler.com) and set up each home computer to have it's own database (metadata).
The replication of the metadata across all of the PCs could get interesting. But if you designate ONE of the PCs as the 'metadata master' where new data (music) is input to the shared network drive (and metadata created) and discipline yourself to use the other PCs for playback only, it shouldn't be too hard.
By the way, both naslite.com and openfiler.com have said that they expect to add UPnP streaming to their NAS software 'Real Soon Now'. I would expect that naslite.com will get there first. Nice thing about using a NAS box as a UPnP server is that it's just there, sitting anywhere in the house at any point on the network, usually admined by a secure web interface. It's a 'headless', closed box that ONLY takes care of disk and streaming. No PC monitor/keyboard/mouse to worry about, etc, etc. It **can** be a PC, but doesn't have to be.