The purpose of this study is to see how beginning or changing anti-HIV medications affects the body composition (weight, height, growth, body fat, and muscle mass, or fat and muscle distribution) of HIV-infected children. This study also looks at how changes in body composition relate to changes in viral load (level of HIV in the blood), CD4 cell counts, height, and weight in HIV-infected children. This study also compares changes in body composition to levels of cytokines (proteins in the body that affect some immune cells) in HIV-infected children who are beginning or changing anti-HIV therapy.