If you can find an exact Windows version 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 info in this tutorial to create an "all versions" installer of any version Windows installer, to install using the same 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