The purpose of this study is to evaluate any differences in the effectiveness, safety, and tolerability of PREZISTA (darunavir; DRV) 600 mg, administered with ritonavir (RTV) 100 mg twice a day on virologic response (defined as a viral load (VL) of < 50 copies/mL) over a 48-week treatment period in HIV-positive women and men. Additional antiretroviral (ARV) agents will also be administered and will be chosen by the Investigator based on resistance testing and prior treatment history (referred to as the Optimized Background Regimen (OBR)).