A Study to Compare Three Doses of T-20 When Given in Combination With Abacavir, Amprenavir, Ritonavir, and Efavirenz to HIV-Infected Adults

Inclusion Criteria Patients may be eligible for this study if they: * Are HIV-positive. * Have an HIV level (viral load) between 400 and 100,000 copies/ml at the screening visit. * Have taken at least 1 PI (protease inhibitor) for at least 16 weeks, and have had no interruptions in their most recent PI-containing anti-HIV drug regimen. * Are at least 18 years old. * Agree to abstinence or use of 2 effective methods of birth control, including a barrier method, during the study. Exclusion Criteria Patients will not be eligible for this study if they: * Have ever taken an NNRTI (nonnucleoside reverse transcriptase inhibitor). * Have an opportunistic (HIV-related) infection. * Have had an unexplained fever of at least 38.5 C for 7 days in a row within 30 days prior to screening. * Have had diarrhea lasting at least 15 days within 30 days prior to screening. * Have ever taken abacavir, amprenavir, or efavirenz (ABC, APV, or EFV). * Have certain genetic characteristics (drug resistance mutations) that could change the way a drug acts in the body. * Are allergic to any of the study medications. * Have a tumor other than certain skin or cervical cancers. * Are on chemotherapy that cannot be discontinued during the study. * Are taking an investigational drug within 30 days prior to screening. * Have ever received an HIV vaccine. * Are taking certain medications. * Abuse drugs or alcohol. * Have hemophilia or another blood clotting disorder. * Have had an organ transplant. * Are pregnant or breast-feeding.