The trial is an open-label study in patients with HIV-1 infection who are naive to treatment with NNRTI and protease inhibitor classes of antiretroviral drugs and have <= 6 months prior exposure to d4T at the time of screening. Part I of this trial is an investigation of potential pharmacokinetic interaction between nevirapine and nelfinavir in HIV-1-infected adults treated with d4T. Part II is an investigation of the long-term antiviral activity of the combination of nevirapine and nelfinavir on viral load as measured by HIV-1 RNA.