Windows 7 How to boot cd from external usb dvd rom

My LAPTOP DVD ROM GOT SPOILED SOME TIMES BACK SO I BOUGHT AN EXTERNAL PORTABLE USB DVD WRITER/READER (DVD DRIVE). NOW THE PROBLEM IS I AM HAVING SOME SOFTWARE WHICH REQUIRES TO BOOT FROM CD ROM. PLEASE HELP ME HOW TO BOOT FROM MY EXTERNAL PORTABLE DVD ROM. I AM USING WIN 7 ULTIMATE 32 BIT

THANKS IN ADVANCE