One way is to get a Squeezebox Receiver and install Squeezebox Server on your PC. You can then stream wirelessly around the house. The receiver is usually bundled with a controller as the Squeezebox Duet, but if you have an iPod Touch or an iPhone, you can get the iPeng app to control it.
This may seem overkill for connection to one stereo. The advantage of the Squeezebox approach is that you can add multiple receivers (other clients, see below), and either stream music to each receiver individually, or synchronise them to get multi-room audio.
There are a few other Squeezebox products you might want to consider too.
I have Squeezebox Server running as a service on Ubuntu my Acer Revo and love it, see this question on my SE site for more details.