LPV/RTV has shown significant antiviral activity and tolerability in clinical trials in adults and children over 6 months of age. LPV/RTV has been approved by the FDA to treat older children and adults with HIV. Data for children under 6 months, however, have not been available. LPV/RTV has not been approved for infants less than 6 months old, and the appropriate dose for young infants is not known. Dosing guidelines are needed for young infants, most of whom are in the early stages of primary infection. This study will help identify an appropriate dose range of LPV/RTV and evaluate response to therapy in infants less than 6 months of age. The study will also evaluate whether early therapy allows normal development of the immune system.