This study is designed to assess the tolerability of different forms (liquid, capsules or tablets) of lopinavir/ritonavir given once-daily as part of combination therapy for HIV infection. Study subjects will be those tolerating a stable regimen of HIV medications with undetectable levels of HIV in their blood. They will be assigned by chance to receive once daily liquid or soft gel capsules of lopinavir/ritonavir for up to four weeks. At that time they will receive the alternative formulation for up to four weeks. They will then be given once daily lopinavir/ritonavir in the recently released tablet formulation. After up to four weeks of each of these formulations several assessments will be made of the overall tolerability of the drug. After four weeks of tablets they will be allowed to take whatever regimen they want and will be followed for an additional 36 weeks for a total duration of study of up to 48 weeks. The pharmacokinetics of each formulation of lopinavir/ritonavir given once daily will also be assessed in a subset of study subjects.