Actually you don't need relax or any other streaming software on your computer to run shoutcast. You can run shoutcast directly from your xbox with xbox media player just click "My Music" then click "Shoutcast"
But before you do that you have to configure the shoutcast directory with some .sc or .pls files that actually work... I don't think that the one that comes with the XBMP program works.
an easy way to get pls files is to use you PC and go to
http://yp.shoutcast.com and search for the genre that you like and RIGHT click on the "Tune In!" Button and click "Save Target as" and save the .pls file to your computer. Rename it something easy to recognize like "80s-station.pls"
After you get that file on your computer FTP it into your "XBMP/SHOUTcast "directory wherever it is on your Xbox
Click on "My Music" then "Shoutcast" then the file you just saved... if your internet connection is good then you should hear some music.
On another note...
For my Box I had to manually put in my ISP's DNS numbers because my router was/is not set up to serve DHCP and when you use a static IP on the Xbox you have to manually enter the DNS (Nameserver) numbers. Remember gateway and nameserver is not the same thing. Gateway would be your router (192.168.0.1) and your nameserver would be something outside of your subnet. An easy way to find out these numbers is go to your PC and open a dos prompt any type "ipconfig /all". Look for the numbers next to the "DNS Servers" these should be you ip address(es). You can enter it in the <nameserver> section.
~Sy