I bought a little box (Hauppage MediaMVP) that hooks up to your TV and your home network that allows you to stream video and music from your computer to your TV and stereo.
I bought the hardwired version. There is a wireless version, but my router is close to my TV and had an open port, and I thought it would work better running wired, based on some reviews (also it requires 802.11G and I have 802.11B).
The server software that comes with the MediaMVP is really awful, constantly froze up, so I ended up buying a copy of SageTv with support for MediaMVP, which works like a champ. SageTv is a bit of overkill, since it supports PVR cards with scheduling and full DVR functionality. For me I am just using it to play downloaded AVI files. I used to download AVI's and burn them to DVD's, which was time consuming (4 hours to convert a 2 hour movie), and wasted a disk.
You need a decently fast computer as your video file server, but the MediaMVP box does the decoding onboard. It has a CPU and starts a version of Linux internally. I think the SageTv software replaces the Linux it comes with, though. You can do all of what SageTv does using open-source software and a lot of time, but I just wanted to watch my TV, not play with software.
This is a really good solution if you want to watch AVI files on your TV, not on your computer screen, but don't even bother to try to get the MediaMVP software to work.