Here's some interesting info you may find useful, you can do a clean install using only a downloaded ISO without creating bootable media; and be sure to post back with any further questions you may have and to keep us informed.
Clean Install Windows 7 from the Windows 7 Desktop
After you have
copied out or made
back-ups of the data you need to save to external media. I
If you can find, or download an exact Windows version installation media that the PC shipped with you can do a
clean install using the activation key on the COA sticker attached to the PC though it may require a robo-call to MS to sort the OEM batch-key.
How to Activate Windows 7 by Phone
If you can't find an exact same version, you could use the information in this tutorial to create an "all versions" installer of any version Windows installation media to install your version using the same activation method I posted above.
Windows 7 Universal Installation Disc - Create
After you have made backups of everything you care to save, the best method is to do a complete wipe
secure erase of the entire Hard Disk Drive first, it over-writes everything, all the old Windows code including all the old
drivers/programs, giving you the best possible space to install Windows to.
SSD / HDD : Optimize for Windows Reinstallation .