Hello RDR, welcome to Seven Forums!
Here's a lot of useful information, have a thorough look before you decide how to proceed and be sure to post back with any further questions you may have and to keep us informed.
There is nothing that should be keeping you from doing a clean install of Windows 7 to that PC, be sure to set the BIOS back to the default if you have made any changes there.
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 
