Since being targeted by a virus I'm told my copy of windows is not genuine

My PC recently became infected with a Trojan Hider, I am in the process of attempting to fully remove the malware and have recovered much functionality that was initially lost such as access to forums/virus software/microsoft.

However, in the process of my attempt at repair my desktop wallpaper was blacked out and replaced with the message 'your copy of windows is not genuine'. On restart I am asked to re-enter my product key. I have just obtained my product key and went to input

it Control Panel > System but the bottom section just says Product ID: Not Available with no input option.

The error code on start up is 0xC004F014 which I know is telling me there is no product installed. What I am unclear about is:

1) Is this error likely to be a result of my human error in trying to resolve the virus issue (without necessarily knowing what I'm doing)

2) Is it likely to be a symptom of the virus

3) Should I reactivate Windows with my genuine product key and if so, how?

Thanks in advance

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