This is a 2-part Phase I/II, open-label trial in HIV-infected infants. Part A assesses the triple combination of ZDV, 3TC, and NVP. Four to eight patients are enrolled in each age cohort (Cohort 1: at least 15 days, no more than 3 months; Cohort 2: over 3 months, no more than 2 years). Part B assesses the quadruple combination of ZDV, 3TC, NVP, and ABC. Eight patients are enrolled in each age cohort (Cohort 3: at least 30 days, no more than 3 months; Cohort 4: over 3 months, no more than 2 years). [AS PER AMENDMENT 3/11/98: This study is now a 3-part Phase I/II trial. Parts A and B are as above. Part C will assess the quadruple regimen of d4T, 3TC, NVP and NFV. Up to 8 patients will be enrolled in each age cohort (Cohort 5: at least 15 days, no more than 3 months; Cohort 6: over 3 months, no more than 2 years). If 3 of 4 patients in either cohort of Part B do not achieve plasma RNA less than 1,000 copies/ml after 16 weeks of quadruple therapy, enrollment of patients to that cohort will stop and enrollment of 8 patients to the corresponding cohort in Part C will begin. For Part C, patients whose RNA level is no more than 1,000 copies/ml at Week 16 will remain on assigned treatment until Week 104. If at any time between Weeks 16 and 104 a patient's RNA level increases to greater than 1,000 copies/ml, plasma RNA will be repeated within 1 week. If both RNA levels are greater than 1,000 copies/ml, the patient will discontinue study treatment and be followed every 12 weeks for 1 year.] [AS PER AMENDMENT 4/14/99: The study has been extended for an additional 96 weeks for children with continued suppression of viral replication (RNA less than 400 copies/ml) at Week 104. If at any time between Week 12 or 16 and Week 200 a patient's RNA level increases to greater than 1,000 copies/ml, plasma RNA will be repeated within 1 week. If both RNA levels are above 1,000 copies/ml, the patient will discontinue treatment for best available therapy and be followed every 12 weeks for 1 year following the discontinuation of study treatment.] [AS PER AMENDMENT 9/16/99: An additional cohort (Cohort 7) of 5 to 10 patients has been added. Cohort 7 includes patients between 15 days and 3 months of age. Cohort 7 patients who experience suppression of viral replication at Week 104 are followed through Week 200.]