Hello tovarplmdl,
If the product key of the Windows platform installed is passing validation then installing Service Pack 3 should not require a key.
It will only require a key if it was not validating with the installed key.
To check the validity of the operating system on your machine please go here: http://www.microsoft.com/genuine.
The genuine site can also provide you with valuable information so please take time and review the documents.
Another site for helping customers identify genuine vs. non-genuine software is located here: http://www.microsoft.com/resources/howtotell/
I hope this has helped!
Lori MS