Infants between 14 days and 6 months of age will receive LPV/RTV in combination with 2 nucleoside reverse transcriptase inhibitors (NRTIs) chosen by their physicians. Twelve-hour pharmacokinetic sampling is performed on Day 14 of drug treatment and when the patient reaches 12 months of age. Patients will undergo a physical exam, medical history assessment, and blood collection at selected study visits. Study visits will occur every 2 weeks for the first 8 weeks, then every 4 weeks until the end of the first year of the study. Study visits during the second year will be every 12 weeks until the end of the study. The parent or guardian will be contacted by phone every 6 weeks to monitor adverse drug effects. Participants between the ages of 6 weeks but less than 6 months old will be followed for 96 weeks after the enrollment of the last participant. Participants between the ages of 14 weeks but less than 6 weeks will be followed for 48 weeks after the enrollment of the last participant.