Computer won’t boot to cd-rom

I have a computer that I removed the hard drive and scanned. It had 40 infections. They were removed. The motherboard was bad, (asus-p4p800) I replaced it with a intel 915gev. Everything shows in the bios, the hd, the cdrom, etc. I try and boot normally, it goes to the windows xp logo and flashes a blue screen error reeaal fast and reboots. I try safe mode, reboots, I set the bios to cdrom first, and put in a xp disc, I never get the option to click any key to boot from cd. It just asks for normally, safemode, or last good configuration, and then reboots. I want to format and reload windows but it won’t let me. Did I skip a step or what?
Thanks
Mark