I think he understood you quite well, if you want to format your PC then Windows will be deleted too. Don't worry, if you have a legit serial key you can just get a free windows .iso file, put it on a spare USB and install it that way. You don't need the disc.
I'll guide you through the process:
1. Go to the link RunLuke provided, scroll to the bottom of the page and click "Download Tool Now" - 32bit if you have a 32bit system, 64bit if you have a 64bit system.
2. When the tool downloads, open it up, click "Run" and wait for it to open.
3. On the first menu, select "Create installation media for another PC" and then click Next.
4. Select your language, the Windows 10 edition that your serial key is for (Home, Professional, etc.) and then select the Architecture - again, if your current windows 10 version is 64 bit, select that, if it's 32 bit, select that.
5. Next you need to choose which media to use - if you have a USB with at least 3GB of space then select "USB Flash Drive", if you don't then select "ISO file". Remember that for the ISO file you will need a CD with the same amount of space as mentioned before.
6. When you've selected one of those, insert the aforementioned media device and install Windows 10 onto that. I'm sure you'll figure it out.
For the format and installing Windows 10 again, I'm pretty sure you know how to, insert USB, if it doesn't do it automatically then change boot priorities in the BIOS, bla bla.
If you don't know how and need further help feel free to tell me