SBS2003 is not officially supported on S1200BT. It's highly recommended to choose an OS from the Tested operating system list.
To workaround your problem, try the following ways:
1. Use a USB floppy drive in Microsoft support list: http://support.microsoft.com/kb/916196
2. Slipstream the driver into the installation CD: http://www.intel.com/support/motherboards/server/sb/CS-029550.htm