There is a way to keep the branding and OEM slic activation. Download the matching ISO for your currently installed version. Then us a program like ultra ISO to add the $OEM$ folder that's on the Dell disk to the sources folder in the ISO you downloaded. You can also copy over and replace the ei.cfg file, that will guaranty the correct version is installed. Then burn a disk from that ISO image. If its done correctly the burned ISO will not ask for a product code and activation will be automatic. Just like the Dell disk would do if it actually worked.
Another way to do it is to use the windows 7 DVD download tool to make a bootable USB thumb drive from the downloaded ISO. Then just copy the ei.cfg file and $OEM$ folder over from the Dell disk to the sources folder on the thumb drive.