Is the buyer, the seller, or both accountable for an unoriginal Windows copy on a used computer that was bought, and is it obligatory to stop using it upon realizing that?

There is no need to investigate the originality of the Windows copy; the default assumption is its legitimacy. However, if it is known to be non-original, our fatwa prohibits its use. Yet, some scholars permit benefiting from it for personal use only, similar to Ibn 'Uthaymeen's view that copying for personal use is permissible, but not for commercial purposes.