First and foremost, you have to have a valid Win 7 product key for installation.
Did the Win 7 come pre-installed in the Dell computer ?
If yes, the product key is an OEM key. It is NOT transferrable.
Which means, you'll have to buy a new license.
If you buy a new license, it will come with an Installation DVD and a new product key. That makes your original Dell Win 7 disc irrelevant.
Besides, it is not usual that a computer with pre-installed Win 7 also comes with an installation disc. It could be a recovery disc which will be no good for new installation.
So, if you provide more info..........