There were two parts to this study. Step I took place in the United States and South Africa, and were further divided into two sets of groups, Parts A and B. Part A participants received ATV only and Part B participants received ATV with low-dose RTV boost. All participants received ATV once a day with 2 other antiretroviral drugs (not provided by the study). In Part B only, participants received ATV with a low dose of RTV. Participants were placed into 1 of 8 groups (Groups 1 to 4 for Part A; Groups 5 to 8 for Part B) with respect to age and study drug formulation. Participants in Groups 1 and 5 were infants between ages 3 months and 1 day (91 days) and 2 years (less than or exactly 730 days) and took ATV in powder form. Participants in Groups 2, 3, 6, and 7 were children between 2 years and 1 day (731 days) old and 13 years old. Groups 2 and 6 received ATV in powder form, while Groups 3 and 7 received the capsule form. Patients in Groups 4 and 8 were adolescents between 13 years and 1 day old and 21 years old (not including the 22nd birthday) and took ATV in capsule form. As of 01/02/2008 a new group, 5A was opened for enrollment. Participants in Group 5A were between 3 months and 6 months old and took ATV in powder form plus a low-dose RTV booster.