Antiretroviral therapy for HIV, particularly with the use of PIs, is associated with an increased risk of heart attack. Specific cardiovascular side effects seen with the use of some PIs include insulin resistance, abnormal blood lipid levels, and endothelial dysfunction (abnormalities in the cells that line the inner surface of blood vessels). Past studies have shown that treatment with indinavir, an older PI, results in significant endothelial dysfunction, which may be the main cause for the increase in cardiovascular risk. Indinavir is now seldom used in clinical practice, and the newer PIs atazanavir and combination lopinavir/ritonavir now account for nearly 70% of total PI use in the United States. It is not known what effect these new PIs have on endothelial dysfunction. This study will compare the effects of atazanavir, lopinavir/ritonavir, and placebo on certain cardiovascular disease risk factors, including abnormal glucose metabolism and endothelial dysfunction, in healthy people without HIV.