This study will examine how the anti-HIV protease inhibitor lopinavir/ritonavir (Kaletra® (Registered Trademark)) affects the function of the endothelium (lining of the arteries). Medications such as protease inhibitors can dramatically change the course of HIV infection in many patients; however, among their side effects is development of abnormal lipid levels resulting in high cholesterol and insulin resistance. These side effects may damage the lining of the arteries that supply blood to the heart, leading to premature coronary artery disease. The study will determine whether lopinavir/ritonavir directly affects endothelial function and whether it alters cholesterol levels, glucose tolerance, and markers of inflammation in people who take the drug for 4 weeks.