A Comparison of Atazanavir and Nelfinavir, Each in Combination With 2 NRTIs, in Patients Who Have Failed Treatments Without a Protease Inhibitor

Inclusion Criteria Patients may be eligible for this study if they: * Have a viral load of 1000 or more copies/ml within 3 weeks before randomization. Have a CD4 cell count of 50 or more cells/mm3 within 3 weeks prior to randomization. * Are at least 16 years old (or the minimum age by local requirements). * Have had more than 16 weeks of therapy with a non-nucleoside reverse transcriptase inhibitor (NNRTI) and/or nucleoside reverse transcriptase inhibitor (NRTI). * Use effective barrier method of contraception. * Give written informed consent. * Are available for follow-up for at least 52 weeks. Exclusion Criteria Patients may not be eligible for this study if they: * Have used a protease inhibitor (PI) treatment for more than 7 days or within 30 days prior to screening. * Have a newly-diagnosed HIV-related infection or any medical condition requiring relatively short but intense therapy at the time of enrollment. * Have hepatitis in the 30 days before the study. Patients with long-term hepatitis are eligible if their liver function meets certain requirements. * Are unable to demonstrate responsiveness to a provided NRTI. * Have had previous or expect a need for therapy with agents that may cause damage to nerve tissue, the pancreas, the liver, bone marrow, or cells within 3 months of study start. * Use too much alcohol or drugs to be able to follow the study therapy; or if they use enough to increase the risk of developing pancreatitis or chemical hepatitis. * Have severe diarrhea within 30 days prior to study entry. * Are pregnant or breast-feeding. * Have a history of hemophilia. * Have history or signs of bilateral peripheral neuropathy. * Have cardiomyopathy. * Have certain heart problems. * Cannot tolerate oral medication. * Have any other problems that would interfere with the study.