One approach to developing a preventive HIV vaccine includes the use of a prime-boost vaccine strategy. This type of strategy involves two vaccines, given sequentially at different time points. The goal is to stimulate different parts of the immune system and enhance the body's overall immune response to HIV. In this study, participants will receive two HIV vaccines 3 months apart. Heterologous-insert prime-boost vaccine regimens, which use the same gene from different HIV-1 subtypes, may be more effective than traditional homologous insert prime-boost vaccine regimens at eliciting immune responses directed at epitopes that are highly prevalent, possibly leading to a more effective immune system response to the vaccine. The purpose of this study is to assess the safety and immunogenicity of a heterologous-insert prime-boost HIV vaccine regimen that uses inserts from different HIV-1 subtypes and different adenovirus vectors.