You probably don’t need to buy a new Windows key just yet. If Windows was activated before the factory reset, it usually has a digital license linked to your laptop, and it should reactivate automatically when you connect to the internet, as long as you reinstall the same Windows edition.
Go to Settings > System > Activation and check the activation status and any error code. If it still says Windows isn’t activated, select Troubleshoot and follow the steps. You can also check whether you installed Windows Home or Pro, since the edition needs to match your original license. See Microsoft's activation troubleshooting guide.
As for PowerPoint, Windows activation and Office activation are separate issues. Try opening PowerPoint through Microsoft 365 or Office and check whether it asks you to sign in or activate Office.