Your post is a bit confusing.  First you stated you want windows 7 for your E5450, however in point 3 you stated that the computer came with windows 7 and a disc for windows 8.1.

If your computer came with windows 7 you don't need the ISO for anything.  The disc for 8.1 is upgrade/downgrade rights discs normally given to business class computers who may not want 8.1 or windows 10.  As for why its on DVD and not USB that I can't tell you.  

If you go the Dell website you posted, input your service tag, and they have an ISO available it SHOULD be the version of windows your computer came with, along with drivers etc.  That being the case, if its windows 7, you shouldn't need the product key for it as its a DELL OEM key that is used and the software makes sure your on a Dell computer.

If Windows 7 came preinstalled you should update Dell Backup and Recovery to the latest version and then use it to create Recovery Media (requires a 16 GB USB flash drive):

http://dellwindowsreinstallationguide.com/dell-backup-and-recovery-1-8-1-71/

Hi beamermt79.

Yes, my dell came with win7 preinstalled and win 8.1 on a dvd.

(to do what with when I have no dvd player?).

You say I dont need the iso? Why?

The iso with win7 is exactly what I need to reinstall win7 when needed.

If I understand you correctly the iso I get from dell will install without me needing product key etc.

So, I guess case is closed then.

Thanks