If you clean install Windows 10 (which often gives you a better result on problematic systems) instead of doing a simple in-place upgrade, you can run into problems activating.
This is usually painless, but you can't do it online. If you've upgraded to Windows 10 and then made major changes to your hardware – especially changing your motherboard, which counts for a lot of points in the scheme Microsoft uses to check if you're still using the same PC – then you'll have to activate by phoning Microsoft. If you want to join the preview now, you have to upgrade to the release version of Windows 10 first and activate it, and then change your updates to Get Insider Builds. If you were part of the Windows 10 Insider Preview before launch, you can't use the same product keys that you might have had for the technical preview they won't even activate old builds any more.
