To play the full version on PC today, you must purchase it or access it through a subscription service.
You can find the original version on several major digital storefronts: dead-space-1-game-free-download-for-pc-full-version
The original is no longer available as a free download. While it was previously offered for free via Electronic Arts' "On the House" program on Origin in 2014 and 2018, that promotion has long since ended. To play the full version on PC today,
If you prefer not to buy the game outright, you can play it through the following services: dead-space-1-game-free-download-for-pc-full-version