tech guy says that my windows is genuine?

Are you sure you don't presently have a validated activated Win 10 installation?
Have you checked? What makes you think you haven't?

Note: upgrades from e.g. Win 7, Win 8 are still free...

- your options include:
a. Use a valid Win 7 or Win 8 key that has not previously been upgraded
b. Buy a license
c. Transfer a Win 10 license from an old Win 10 PC

miroplesko said:

what's best way to get permanently activated windows 10 then? should I buy it? Is there a way to convert my current OS to a pen drive ISO?

What is your current OS?
If it is Windows 10, what does it say in Settings, Updates, Activation?

If it says that your Windows 10 is 'activated with a digital licence' then you can reinstall Windows 10 at any time in the future.
- Your computer will then re-activate automatically by quietly contacting the MS activation servers & checking their records of your current activation.
- All this will happen without you having to do anything.
- You could even install Windows 11 instead if you wanted to. A Windows 11 licence / activation is exactly the same as a Windows 10 licence / activation.

This is how activation is managed.
- The ISO file itself has nothing to do with the licence or with activation.

When you reinstall Windows 10, you should use this procedure - Clean Install - TenForumsTutorials


Best of luck,
Denis