Windows 7 ISO image

Hello,

I need to reinstall a clean copy of windows 7 Pro x64 on my Optiplex 745. I have the product key and service tag #. How do I go about downloading a new ISO image? I want to burn the image to a dvd, boot from disc drive and do a clean install. 

Please help!

Update:

Please watch this video for help:

This link will help if the system is not booting and the OS needs to be reinstalled.