A Multi-Center, Placebo-Controlled, Double-Blind, Randomized Trial Comparing the Virologic and Immunologic Activities of 400 Mg Nevirapine in Combination With Zidovudine Versus Zidovudine Alone in Asymptomatic HIV-1 Infected Patients With 4-12 Months of Prior Zidovudine Therapy and 200-500 CD4+ Cell