a boot disk from dvd drive? I've had to format the hard drive, and when I try to load Windows on the computer, it bypasses the dvd drive (even though I have it first in the boot order).. Any help would be appreciated

Since I formatted the drive to remove the virus completely I need to re-install my copy of Windows. I can't do it because during boot up, the dvd drive light comes on, but instead of running the Windows disk, it goes to hard drive and saying "BOOTMGR is missing. Press Ctl-Alt-Del to reboot". How can I boot from a flash drive or external hard drive?