All of the Windows 7 installers are the same - retail or OEM - with other versions hidden. It is the key which determines which will activate.
Where did you get the Windows 7 DVD? Is it a Dell Reinstallation DVD? Clean it to make sure it's useable.
If not download a new ISO for your licensed version from the Official downloads list, then burn to DVD or write to flash stick using the tool given in these steps to get a perfect OEM reinstall: Clean Reinstall - Factory OEM Windows 7
If it continues to fail, then wipe the HD from DVD System Recovery Options
Command Line using Diskpart Clean Command
. Report back what happens and at what stage.