Monotherapy phase: To evaluate and compare the safety, tolerance, pharmacokinetics, and preliminary activity of nevirapine administered alone in mildly to moderately symptomatic HIV-infected children ages 2 months to less than 18 years; to evaluate and compare the safety, tolerance, and pharmacokinetics of nevirapine in HIV-infected children ages 1 day to less than 2 months. Combination therapy phase: To evaluate and compare the safety, tolerance, pharmacokinetics, and preliminary activity of nevirapine administered in combination with zidovudine (AZT) in mildly to moderately symptomatic HIV-infected children ages 2 months to less than 18 years.