I'm not familiar with Abit motherboards, but there should be a 'boot' menu when you POST, allowing you to boot from CD/DVD, floppy disk, hard drive, USB etc. You need to boot from CD and then install Windows 7 or whatever OS you like. After you install Win7, it should fix up the boot priorities on your PC to make sure it boots and not any other OS.
Hope that kind of makes sense to you...