Ok, I don't know which mobo you're using but what you want to find is the boot order. This should definitely be in the user manual.
It will most likely be in the order of Floppy, HDD, and then CDROM
Change it so the CDRom is booted first, save and then exit. This will exit BIOS, reboot the PC and then boot to your Windows install disc.
If you haven't already I'd really suggest doing a low level format of your HDD before you get started (use something like killdisk)
Another thing to check is if you have two CD rom/ DVD drives. If one is not working, just try it again with the other.