May be an OEM disk made for HP may not work for you.
It happened to me once. I tried a Compaq Win XP OS installation CD (Not a Recovery Disk) on my office IBM thinkcenter desktop. The setup program specifically told me that this particular setup CD is meant for Compaq only and the setup has aborted. Some of the manufacturers make a custom made OS installation disks and make sure they do not work in other systems.
If you have a generic OEM DVD then it may work. You may provide your windows 7 key on the Microsoft certificate pasted on your system. If it is not activated automatically, you may have to call Microsoft for telephone activation.
Your best bet would be to get hold of a retail copy of windows which will work on any system. After installation of windows use the same key from your microsoft sticker on your system as explained above.
Afterwards you can go to the Dell site and download any drivers
which Windows 7 has not natively installed.