The purpose of this study is to find out whether taking protease inhibitors (anti-HIV drugs) together with lipid-lowering drugs (drugs which lower the amount of fat in the blood) has an effect on the level of drugs found in the blood compared to when these drugs are taken separately. The three protease inhibitors given in this study are ritonavir, saquinavir, and nelfinavir. The lipid-lowering drugs given are pravastatin, simvastatin, and atorvastatin.