First make your Dell Recovery Disks off of the Recov partition, if you don't have them already.
Next back up all of your files by dragging folders onto external or DVD. You can also back up windows/system32/drivers
and driverstore files so you know you will have all of the drivers if any are missing and unavailable on Dell's website.
Now boot the Windows 7 DVD, select Custom install, use Drive Tools to delete all partitions, create New one(s) as you wish, then format before installing to first partition.
The installer is mostly driver
complete, with newer ones arriving quickly via optional Windows Updates. If any drivers are then missing, find them plus favorite apps (cam, fingerprint, etc) on Dell's website Support page for your model.