Follow these steps to get a perfect Clean Reinstall - Factory OEM Windows 7
with Special Notes to Dell Owners at end to understand how the OEM partitions are best handled.
The "Realtek PXE media test failure" error prompt sounds like the BIOS in trying to boot into a Network choice instead of HD. Make sure HD is set first to boot in BIOS setup. Then trigger the DVD or flash stick installer using Dell F12 key.
Do you still have Dell Diagnostics available from F12 menu? If not then you can delete all partitions before reinstall since you already have lost Recovery functionality if not content.