Control of the global HIV pandemic will require the development of a safe and effective vaccine. Many HIV preventive vaccines that are in development use a recombinant adenovirus serotype 5 (rAd5) vector as a way of delivering the vaccine into cells. However, the majority of people, particularly in the developing world, have immunity against the Ad5 vector, which means the vaccine will not be effective at preventing HIV infection. This study will use a rAd26 vector as part of a HIV vaccine, as research has shown that relatively few people have immunity against this vector. The purpose of this study is to evaluate the safety and immunogenicity of a rAd26 vector preventive HIV-1 vaccine.