The first Witcher game is currently available for free to players with a GOG.com account. In an effort to promote the spin-off game derived from within the Witcher trilogy, Gwent, GOG.com has included an enhanced edition of the first game as a signing-on bonus of sorts. If you have a GOG.com account, follow this link
and click on the big 'Subscribe and Claim' orange button to secure your free copy. Once you have downloaded the site's client, GOG Galaxy, you can then proceed to subscribe to their official newsletter. As part of the promotion, you will also need to add the game Gwent to your GOG.com games library (an installation is not required). This game is also free. After completing the aforementioned steps, the enhanced edition of the first game is yours to try and keep.