Hi
You may need to change the boot order in the bios to make sure that the DVD drives is first in the boot order.
You have to write the ISO file to a DVD using a program like ImageBurn, (what I've always used) so that it's bootable, if you didn't do that.
The Official ImgBurn Website
Follow these instructions.
Clean Install Windows 7 - Windows 7 Forums
Make sure you back EVERYTHING up first, that means, Fonts, Address Books, Favorites, Documents, and all saved data.
You will have to reinstall all of your software afterwards.
Mike