The purpose of this study is to look at the effect of giving 3 and 4 study medications together on the amount of HIV virus in a child's blood. The combination of 3 medicines is zidovudine (ZDV), lamivudine (3TC), and nevirapine (NVP). The combination of 4 medicines is ZDV, 3TC, NVP, and 1592U89. The goal is to see if subjects will have an undetectable amount of HIV virus in the blood by week 12. Another purpose is to see how long after week 12 the amount of virus in the blood remains undetectable. This study will also determine the safety of 3- and 4-medicine combinations.