Cohort I: The purpose of this study is to see how safe it is to combine 2 anti-HIV medications, efavirenz (EFZ) and nelfinavir (NFV) to treat HIV-positive children and to find an appropriate dose of EFZ to use in combination with NFV. Cohort II: The purpose of this study is to see how safe it is to give EFZ syrup combined with NFV and to measure the levels of EFZ and NFV in the blood. (This purpose reflects a change from the original since there are now 2 different cohorts of patients.) EFZ is an effective anti-HIV medication that easily can be combined with other drugs to treat HIV. This is an early study to determine a safe and effective dose for HIV-positive children. This study also will examine the correct dose of NFV to use in combination with EFZ.