The shader model compatability is built in to the hardware, sadly it's not something you can just download. A new graphics card is the only option if you want to play the game, or, judging by the age of 4th generation nVidia Graphics cards, it may be cheaper and better to buy a brand new PC alltogether.
If you bought this PC less than 5 years ago, I'm afraid you may have been ripped off... Most likely even replacing the graphics card won't be enough, there'll be something else that is too old to run Legacy.