Download Pin Drive Linux and look for DBAN (Darik's boot and nuke), select download iso on the right hand side and install it onto the usb. When it's completely installed, reboot the computer and access the boot order or bios and change the usb to first priority of when starting the laptop. After that is done, perform a complete wipe and wait until it's done. Pm me if you need any help. Cheers.

Here's the link to Pin Drive Linux
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What type of computer is it? Like mine I just push the windows key, type "Dell" and it pulls up some things, the first name is usually different, but it's almost always (Computer Brand) Backup and Recovery, click that and it will guide you through steps, just click the factory restore, I did it awhile back and never downloaded anything.

Scratch what I said previously, as there's an alternative method that is painstakingly easier than the DBAN method. Start by clicking the windows button + C, select more pc settings, general, and select "restart your pc and start over". Hope this helps.