Have a look through this information below to see the best option for your situation and be sure to post back with any further questions you may have and to keep us informed.
After you have copied out
or made back-ups
of the data you need to save to external media.
If you can borrow or download installation media of the exact same Windows 7 version the PC shipped with you can do a clean install
using the activation key on the Certificate of Authenticity (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 7 installation media so you are able to install your version using the same activation method I posted above.
Windows 7 is version specific and not bit-rate specific, i.e. a Home Premium COA activation key will activate either the 32-bit or the 64-bit Windows 7 Home Premium Operating System. Windows 7 Universal Installation Disc - Create ISO - Disk Image File : Extract to the Desktop
After you have made backups of everything you care to save to external media, the best method forward is to do a complete wipe secure erase
of the entire Hard Disk Drive / Solid State Drive first, it over-writes everything, all the old Windows code including all the old drivers and installed programs, giving you the best possible space to do a clean installation of Windows 7 to. SSD / HDD : Optimize for Windows Reinstallation Additional Secure Erase / Wipe Information
For those with netbooks and no CD/DVD drive, have a look at this tutorial. USB Windows 7 Installation Key Drive - Create .