LY517717 (a capsule given by mouth) is a blood thinner that may prevent blood clots from forming in the legs and may prevent those blood clots from traveling to the lungs. Leg and lung blood clots occur commonly after patients have surgery to replace a hip or knee joint. These clots often occur while patients are in bed in the hospital after hip or knee joint surgery. The purpose of this study is to test if different dose strengths (amount of drug in the capsules) of LY517717 can prevent blood clots from forming and to determine if LY517717 is safe. This study will compare LY517717 to enoxaparin, another blood thinner. Enoxaparin is one of the standard medications given after hip or knee joint surgery.