You need first to set your HDD properly by pressing F4 right after POST when prompted, then going on booting from CD.
After pressing F6 when prompted, just insert the Silicon Image floppy provided with your board and Windows Install will ask for it eventually. Just press enter when it asks you to insert the floppy in A: to load Sil3112 drivers. If you don't have such a floppy, download the drivers from
<A HREF="http://www.giga-byte.com/Motherboard/Support/Driver/Driver_GA-7N400 Pro2.htm" target="_new">this site</A> and copy them onto a floppy.