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.