If it has the factory recovery partition installed, you can simply do the following. If it does not and you do not have installation media you would need to locate your installation key using magic jelly bean keyfinder (IF you have a valid licensed version installed. If not, you're SOL.)
This will extract your current license key, if valid, it can be used to reinstall:
https
/www.magicaljellybean.com/keyfinder/
You can also reset your installation for most versions by doing the following, IF, you still have the recovery partition that came on the laptop's hard drive:
http/www.pcworld.com/article/2364269/how-to-reinstall-windows-like-a-pro.html
If you have or are willing to buy installation media (Which is not a bad choice at this time since OEM versions will not qualify you, but purchased versions of Windows 7, 8 or 8.1 will allow you to upgrade to Windows 10 for free when it's released) you can do a clean install using the following methods.
Windows 7 clean install:
http/www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
Windows 8 or 8.1 clean install:
http/www.eightforums.com/tutorials/2299-clean-install-windows-8-a.html
IF you extract your current valid license key using the keyfinder, and need installation media, you can download and use the following to reinstall:
http
/www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/
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