This study examines the antiretroviral activity of ZDV/3TC/NVP in vertically-infected infants and children aged 15 days up to 2 years, and ZDV/3TC/NVP/ABC in vertically-infected infants and children aged 30 days up to 2 years. This study will assess HIV-1 and CD4/CD8 T-cell kinetics, activation, and maturation. It will also test the concept that early (younger than 3 months of age) use of potent, combination antiretroviral therapy will allow the long-term control of viral replication with preservation of normal immune function. [AS PER AMENDMENT 3/11/98: This study will also examine the antiretroviral activity of ZDV/3TC/NVP/NFV in vertically infected infants and children.]