Read over carefully these steps to get a perfect Clean Reinstall - Factory OEM Windows 7
including the Special Notes for Dell Owners.
First run Dell Diagnostics to test all of your hardware, by tapping the F12 key at boot to see if it will run all the way though.
If so and it comes up clear, I would delete all other partitions except Dell OEM Diagnostics partition during booted Custom install to see if that will suffice.
If not you'll need to sacrifice the bootable diagnostics by running Diskpart Clean Command
from the installer's Command line accessed by pressing Shift + F10 at first screen after booting into installer.