For the prime rib roast: Preheat the oven to 450 degrees F.
Place the rib roast, fat-side up, on a rack in a roasting pan. Cut 1/2-inch slits all over the fat cap and insert garlic slices into each slit. Sprinkle the roast generously with salt and pepper. Add the beef stock and butter to the bottom of the roasting pan.
Roast for 15 minutes. Lower the heat to 350 degrees F and roast, basting every 30 minutes, until medium-rare, 3 to 3 1/2 hours. Remove the roast from the oven and let it rest for 15 to 20 minutes before carving. Skim the fat from the juices in the bottom of the pan, and serve alongside the roast.
For the horseradish sauce: In a bowl, mix together the sour cream, prepared horseradish and lemon juice. Season with salt and pepper. Serve with the roast.