Hello mate, here's additional information on the complete wipe / clean install process.

After you have copied out or made back-ups of the data you need to save to external media, then boot the Windows 7 installation media to use Step One of this tutorial at the first link below to do a wipe (secure erase) to the entire Hard Disk Drive / Solid State Drive.
  • Then if you do not want to create the new Windows 7 "System Reserved" partition use the outline in Step Two #2 to create, format and mark Active a single 100GB partition to do the installation to.
  • If you do want to create the "System Reserved" partition use the outline in Step Two #3 to create, format and mark Active the System Reserved partition and then create and format the 100GB partition to do the installation to.
Either way, running the "clean all" then creating and formatting the partition(s) using diskpart will get you the best possible space to do a clean install of Windows 7 to; you can always extend the Windows partition to include the remaining unallocated space on the HDD / SSD or create additional Primary partitions or an Extended partition after the installation completes if you choose.

SSD / HDD : Optimize for Windows Reinstallation

DISKPART : At PC Startup

Do a Clean Install with a Upgrade Windows 7 Version

If you don't want to wipe the entire HDD, you can do a partition-specific wipe of the Windows partition using the Partition Wizard Bootable Disk (PWBD) have a look at Option Three in the tutorial at this link below.

Partition Wizard : Use the Bootable CD