My computer won't boot from DVD drive or USB drive.!!!?

I have a Compaq Presario f750US with an phoenix motherboard. My little brother taught it would be funny to put FORMAT in the command prompt and it erased everything in the hard drive. Now I am trying to install windows 7 on it, but it always tells me BOOTMGR is missing press CTR+ALT+DEL to restart. I changed the BIOS order so it would boot from the DVD drive, and then so it would boot from the USB drive when I was using a external harddrvie. PLease someone hELP!!!!!!!!!!!!!!!