GeexBoX is a nice system that can be used for turning an old computer into a media center. The project has just released a new beta of the coming stable release 1.2. The system boots from CD so it is easy to try it on any old Pentium class II computer with at least 400 MHz CPU and 64 MB RAM.
The ISO image has grown to huge 19 Mb download, but you should be able to download it in some 30 seconds with a decent broadband connection.
Downloads available for 32 bit and 64 bit architectures and even PowerPC from here.
If you have succeeded in turning an old computer into a usable media center, please share your experience in the comment box!