Part A: To determine the safety and pharmacokinetics of sequential single doses of cidofovir in HIV-infected children with end-organ cytomegalovirus (CMV) disease. Part B: To determine the safety (including time to progression of CMV retinitis by retinal exam), pharmacokinetics, and long-term (6 months) tolerance of multiple-dose cidofovir in HIV-infected children with CMV retinitis. Part B: To determine the effect of multiple-dose cidofovir on the virologic parameters of CMV retinitis (viral load, shedding, and resistance to antiviral agents).