Enrollment into Part 2 (Groups 2A and 2B) will begin after the completion of the safety data evaluation of Groups 3 and 4 and after Part A has been fully enrolled. Group 2A participants will receive the DNA plasmid vaccine or placebo at study entry and Days 28 and 56. They will also receive either a low dose of the adenovirus-vectored HIV vaccine or placebo at Day 168.