windows reactivation notice

I just got a notice to update adobe acrobat on my computer which I did, and when I restarted the computer I got the following notice:
" Since Windows was first activated on this computer the hard ware on the computer has changed significantly. Due to these changes, Windows must be reactivated within 3 days"
It then asks me if I want to do so.
I don't know what this means or what to do.
More worryingly , I have MS essentials and it is telling me something about windows not passing a genuine validation test.
There is no pirate software on my computer so I really don't understand this or what to do. Can anyone please help me?

 

Since posting this I followed the steps to re-activate on line and everything seems to be ok now so panic over.
However I don't understand why this happened, can anyone tell me and whether it's likely to happen again.
Thanks.

 

Thanks for that Tim, it seems to me that a few more people should know that since this kind of message is pretty worrying to techno-tyros like myself. I future I will decline any acrobat updates.
Thanks again