This is a Phase III, randomized, double-blind, placebo-controlled trial to evaluate the long-term efficacy, tolerability, and safety of TMC125 as part of an antiretroviral therapy (ART) regimen containing TMC114/ritonavir (RTV) and an investigator-selected optimized background (OBR) in treatment-experienced HIV-1 infected patients. TMC125 is a non-nucleoside reverse transcriptase inhibitor (NNRTI). TMC114 is a protease inhibitor (PI). In this trial, TMC114 will be given with a low dose of ritonavir (RTV), a protease inhibitor commonly used with other, full dose protease inhibitors to improve activity. Additional assessments to be evaluated in this trial include: changes in the HIV-1 genotype, drug susceptibility, and the population pharmacokinetics of TMC125. A pharmacokinetic substudy will be performed at selected sites. Health-related quality of life will be assessed for patients receiving an antiretroviral therapy containing either TMC125 or placebo. Safety and tolerability will be documented throughout the trial. Six hundred HIV-1 infected patients on a stable but virologically failing regimen will be included in the trial. Patients should have at least 1 documented non-nucleoside reverse transcriptase inhibitor (NNRTI) resistance-associated mutation (either at screening or from historical genotype reports), an HIV-1 plasma viral load > 5000 RNA copies/mL at screening, and at least 3 documented primary protease inhibitor (PI) mutations. Patients will be randomized in a 1:1 ratio to either TMC125 (200 mg twice daily) or to matching placebo; both in combination with TMC114/RTV (600/100 mg twice daily) and an investigator selected OBR of at least 2 antiretrovirals (ARVs), consisting of nucleoside reverse transcriptase inhibitor(s) (NRTI[s]) with or without enfuvirtide. The trial will involve a screening period of up to 6 weeks, a 48-week treatment period, and a 4 week follow-up period. Patients will take 200 mg oral doses of TMC125 tablets or placebo tablets, twice daily (administered as 2 tablets twice daily, with food) in combination with 600 mg oral doses of TMC114 tablets and 100 mg oral doses of ritonavir (administered as 2 tablets of TMC114 and 1 capsule of ritonavir twice daily, with food). The treatment period is 48 weeks.