This study will last 1 year. Participants will be randomly assigned to one of two groups. Within each group, participants will be randomly assigned to receive either vaccine or control injections. Group 1 participants will receive either placebo or the adenoviral HIV vaccine at study entry and Month 6. Group 2 participants will receive either placebo or the DNA HIV vaccine at study entry and Month 1 and the adenoviral HIV vaccine at Month 6. Participants will be asked to record their temperatures and other side effects in a symptom log on the day of each vaccination and for 3 days thereafter to report any side effects.