HIV-infected patients 18 years and older who have never been treated for their infection may be eligible for this study. Candidates will be screened with a history and physical examination, including blood tests. Participants will take the following medications once a day: 1200 mg of amprenavir (8 capsules); 300 mg of ritonavir (3 capsules); 600 mg of abacavir (2 pills); and 300 mg of lamivudine (2 pills). Patients will have routine blood tests and be seen by a nurse or doctor, or both, at follow-up visits at weeks 2, 4, 8, 12, and 16; then every 8 weeks until week 48; and then every 3 months for up to 3 years. At week 2, a special blood test will be done over the course of a day to measure blood drug levels. For this test, blood samples will be drawn 8 times over a 24-hour period. A heparin lock (a device that allows the needle to remain in the vein) will be used to avoid multiple needle sticks.