This is a randomized, placebo-controlled, multi-site, parallel-group, double-blind study of vicriviroc maleate(SCH417690) in approximately 120 adult HIV-infected subjects with no detectable X4-tropic virus who have >=1000 copies/mL plasma HIV RNA despite receiving a standard antiretroviral therapy (ART) regimen for at least 6 weeks prior to Screening and at least 8 weeks prior to randomization. Subjects must have previously received >= 3 months of at least 3 classes of marketed antiretrovirals and their HIV must exhibit >=1 resistance mutation to NRTIs and >=1 primary resistance mutation to PIs. Eligible subjects will be randomized to treatment with vicriviroc maleate 20 mg QD, 30 mg QD or placebo and will, in addition, be prescribed an optimized background regimen of marketed antiretrovirals selected on the basis of the susceptibility pattern of their HIV as determined by genotype and phenotype testing, the subject's history of prior antiretroviral use, and drug toxicity. OBT must include at least 3 antiretroviral drugs (not including study drug), 1 of which must be a ritonavir-boosted protease inhibitor (>=100 mg ritonavir). The vicriviroc maleate or placebo doses will be blinded while the optimized background will be open-label. Study visits after Day 1 are scheduled for Weeks 1, 2, 4, 6, 8, 12, 16, 20, 24, 32, 40 and 48. Safety will be monitored by standard laboratory tests, assessment of adverse events and emergence of complicating medical conditions, including AIDS-defining events. Cardiac safety will be monitored periodically by ECG and plasma samples will be collected from all subjects at several time points through the study for population pharmacokinetic analyses. After Week 48, subjects will be offered open-label vicriviroc 30 mg QD until commercially available or until the sponsor terminates the clinical development of vicriviroc. Additionally, subjects who discontinued early from the study prior to Week 48 will be offered re-screening for the open-label segment of the study. During the open-label segment of the study, safety will be monitored by standard laboratory tests, assessment of AEs and emergence of complicating medical conditions. Subjects who terminate participation in either the double-blinded portion or open-label segment of the study will be requested to consent to enter a registry in order to evaluate long-term clinical outcome for up to 5 years.