its not that simple in all cases...not every OS uses a bootable CD
what OS are you planning to install??
in most cases you can just insert a bootable floppy with the necessary files on it (the windows boot up disks work for windows....im gunna figure windows is what you are going with...)
and then go to the start OS with cd-rom support and then it installs some stuff....then when you get to a command prompt (a flashing cursor waiting for imput) you type in setup.exe (with the windows cd in the drive) and you are off...
if its XP (possibly 2k too, i dunno) you can just insert the cd in the drive before it boots up and it should detect it and start up setup
you will have to make sure you have the floppy or cd-rom drive BEFORE the hdd in the start up line in your BIOS....that will tell your computer which it wants to read first (if your hdd is read before your floppy or cd-rom is read, it will give you an error saying something like "no OS detected" but remember, it will give you that error in other cases too)
gl and have fun..... welcome to the forum and to the world of BYO! (building your own)
edit- heh, now that i think of it...i think win98 could even boot from the cd....i dunno anymore... lol