Hello Pentomic, welcome to Seven Forums!
Here's a lot of useful information below, use what you need and disregard the rest and be sure to post back with any further questions you may have and to keep us informed.
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.

Whether you use diskpart to run either the clean all command to completely wipe a HDD / SSD or just the clean command to remove the format configuration data and don't use diskpart to create at least a single partition to do the installation to, letting the Windows installer do the format you will end up with the new Windows 7 System Reserved partition, if you want to make your own decision whether this partition is added, study these two (2) options below.
- 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