As others have suggested, you should download the patches. The latest so far is 1.4 (I think). This is only if you want to play the original game.
I would argue that you shouldn't even bother playing the stock game. Upgrade to the official 1.3 patch, and then download the Stalker Redux/Alive mod bundle. It is the bee's knees.
Redux adds realistic damage and accuracy based on real guns/ammo statistics which a gun-nut modder took weeks to input, and the Alive mod component turns on many of the "A-Life" free-roaming NPC bits which were supposed to be included in the original release, but were cut by THQ to speed up the release.
This mod combo is some of the best fun I've ever had in any game--it becomes very challenging and "alive" (lol) with the extra modding.
Here's the link to the Stalker Redux forums:
linky You should find the proper stuff there--just be sure to click on the "list all" drop down in the releases section, or you will probably not see all of the older files.
Enjoy the game! It was a really pleasant surprize for me after I added the mods, despite the initial missed promises the game didn't deliver upon release.