The study is designed to determine the metabolic changes, antiviral activity, safety, and tolerability of atazanavir as compared to LPV/RTV, each in combination with 2 nucleosides, over the initial 24 weeks, with a final analysis at 48 weeks.

Inclusion Criteria

Patients may be eligible for this study if they:

• Are receiving a PI-containing highly active antiretroviral therapy (HAART) for at least 12 weeks.
• Have a viral load of 1,000 or more copies/ml within 4 weeks of screening and within 4 weeks prior to randomization.
• Have 2 CD4 cell counts of 50 or more cells/mm3 at least 96 hours apart and within 4 weeks before randomization.
• Are at least 16 years old.
• Have a documented virologic response to at least 1 HAART regimen.
• Have phenotypic sensitivity to at least 2 of the following: ddl, d4T, ZDV, 3TC, ABC.
• Have phenotypic sensitivity to atazanavir and LPV/RTV.
• Use effective barrier methods of birth control.
• Will be available for 48 weeks.

Exclusion Criteria

Patients will not be eligible for this study if they:

• Have taken 2 or more PIs and had virologic failure.
• Have taken atazanavir or LPV/RTV.
• Have an HIV-related infection (within 30 days) or any medical condition requiring treatment at the time of enrollment.
• Have had acute hepatitis in the 30 days prior to study entry.
• Have received certain drugs within 3 months of study start or expect to need them at time of enrollment.
• Abuse alcohol or drugs in a way that would interfere with the study.
• Have very bad diarrhea within 30 days prior to study entry.
• Are pregnant or breast-feeding.
• Have a history of hemophilia.
• Use lipid-lowering drugs (within the previous 30 days).
• Have cardiomyopathy or symptoms of other heart disease.
• Cannot take medicine by mouth.
• Have any other condition that would interfere with the study.
• Have pancreatitis, if choosing ddI as part of NRTI pair.
• Have bilateral peripheral neuropathy at time of screening, if choosing ddI or or d4T as part of the NRTI pair.