Before you do that, read this.......
If the initial upgrade is successful and activated, then any subsequent re-install will be activated automatically.
If the initial upgrade is not activated, then clean install will not get activated either.
Your situation is...... you can't even login to find out if the system is activated or not.
If it turns out to be not activated, then your clean install will be a waste of time.
Good thinking. However, by the time I read your post, I have done a clean upgrade (I thought the 1st upgrade with preservation of apps and files would automatically activate Windows 10. No. It did not.)
I then tried to restore the laptop back to the factory installation by holding down Alt-F10 during power up. So, it would boot from the reserved partition. It did not work. Every time I tried it, it boot to Windows 10 after releasing keys.
Well, I did make a set of recovery DVDs when I got the laptop. I put in the first DVD and boot from it. It reported some kind of hardware failure and quit. I suspected it had something to do with Windows 10. So, I installed Windows 7 on the laptop. Once it finished installation, I immediately put in the first recovery DVD and restarted the laptop. This time, it successfully boot from the DVD and I finished restoring the laptop to its factory state.
The next thing I did was run a program to extract the product key because the original product key label is on the bottom of the laptop. It is now partially illegible. I used two different ways to extract the key. Both gave the same key. However, very strange, the extracted product key is different from the one on the label.
Tonight when I go home, I'll first make sure I have sp1. Then, I'll do another clean upgrade.