For PRAM-1: To evaluate zidovudine (ZDV) + lamivudine (3TC) vs. stavudine (d4T) + ritonavir vs. ZDV + 3TC + ritonavir with respect to the change in plasma HIV-1 RNA copy number from baseline to 48 weeks [AS PER AMENDMENT 1/5/98: 72 weeks; AS PER AMENDMENT 7/17/98: 48 weeks] in stable HIV-infected children with >= 16 weeks of prior continuous antiretroviral therapy. To evaluate the safety and tolerance of ZDV + 3TC vs. d4T + ritonavir vs. ZDV + 3TC + ritonavir based upon laboratory and clinical toxicities.