Each volunteer will participate in the study for 36 months. Participants will be randomly assigned to one of two groups. Group 1 participants will receive the DNA HIV vaccine at study entry and at Months 1 and 2. At Month 6, Group 1 participants will receive an injection of the adenoviral vector HIV vaccine. Group 2 participants will receive placebo at study entry and Months 1, 2, and 6. There will be 17 study visits, which will occur at study entry and every 2 weeks thereafter until Day 70; at Month 6 and every 2 weeks thereafter until Day 210; and Months 9.5, 12, 18, 24, 30, and 36. A physical exam, adverse events reporting, HIV and pregnancy prevention counseling, and medication history will occur at each visit. Blood and urine collection will occur at selected visits.