Windows 10 Un-Activated

[eWizard]

So a few months ago I built a new PC and rather than buy a new Windows 10 key, I used one off an old laptop I don't use anymore. Now I want to start using that laptop in online mode again, will I run into any issues with key conflicts? Can I run that old laptop with Windows 10 without registering a new key? 

[IgorM]

probably this message will show up on wor laptop screen 

[demonix00]

If by old laptop, you mean a laptop that had a license key for an older OS (like windows 7), then it's likely that the license on the laptop is still active since the original license would have been forgotten unless the install that was upgraded was the factory install in which case the license on the laptop would still be active since factory installs of windows before windows 8 used a generic install that was already active.