Vicriviroc (vye-kri-VYE-rock) is an investigational drug (not yet approved by Government Regulatory Authorities for commercial use) that belongs to a new class of drugs, called CCR5 receptor blockers. This group of drugs blocks one of the ways HIV enters T-cells (the cells that fight infection). Previous smaller studies in HIV treatment-experienced patients, have shown that vicriviroc is safe and effective. The purpose of this study is to investigate in subjects with detectable dual/mixed CCR5/CXCR4-tropic HIV whether vicriviroc when added to other appropriate HIV drugs can decrease the level of HIV (viral load) in the blood and that it is well tolerated.