This is a 48-week [AS PER AMENDMENT (APA) 12/21/98:104-week] [APA 5/8/00: 208-week] study. It is designed to minimize the chance that ineffective therapy is provided (short dose-escalation phase) and utilizes an area under the concentration time curve (AUC) to establish plasma levels of DMP 266 and nelfinavir in the pediatric population that are both tolerable and efficacious. [APA 5/26/98: Patients are stratified by age into Cohorts I and II] and receive EFV concurrently with NFV.