Back up your data first to run Diskpart Clean Command
from the installer's Command Line accessed by pressing Shift + F10 at the first screen when booted, which will also Convert a Dynamic Disk to a Basic Disk.
If you have data on the HD you wish to preserve or wish to keep the Dell bootable Diagnostics partition intact (Recovery partition will not run after a Clean REinstall) then burn to CD and boot free Partition Wizard v.4.22 which is the last one where the Conversion feature was included free: pw422.zip ISO - Windows Live
Note that you must delete the extra partitions which caused the conversion to Dynamic first. You may only have four Primary Partitions, or three Primary and unlimited Logical data partitions. How to convert dynamic disk to basic disk with Partition Wizard?
Then follow these steps to get a perfect Clean Reinstall - Factory OEM Windows 7