Have you studied the steps in Clean Reinstall - Factory OEM Windows 7
so you understand the the methods and tools to get a perfect reinstall?
If so boot the Dell Reinstallation DVD, choose Custom Install, then delete all partitions except the OEM Diagnosics partition, create New partitions in the space as shown in Steps 7 and 8 of Clean Install Windows 7
illustrated steps, format the new partition(s) only, then click Next to install.
Pay close attention to how drivers are handled in Windows 7 as it is different than XP. The closer you stick to the steps in the Clean Reinstall tutorial, the better install you will have and keep.