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 to.
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