Hello uffypotter.
I would suggest that after you have
copied out or made
back-ups of the data you need to save to external media you use Step One of this tutorial at the link below to do a wipe (secure erase) to the entire Hard Disk Drive, running this then using the outline in Step Two #2 to create and format a single 100GB partition using diskpart will get you a clean all and a
chkdsk /r for the best possible space to install Windows 7 to.
Be sure to post back with any further questions you may have and to keep us informed.
Information
After you have made backups of everything you care to save and you want to do a real wipe
"clean all" to the Hard Disk Drive (HDD) have a look at this tutorial at the link below, it over-writes everything, all the old Windows code including all the old Windows drivers, giving you the best possible space to install Windows 7 to; it goes hand-in-hand with a clean install.
SSD / HDD : Optimize for Windows Reinstallation
How to Do a Clean Installation with Windows 7