Phase IIB, Randomized, Multicenter, Parallel Group, Study to Evaluate the Safety, Pharmacokinetics and Antiviral Effect of Four Blinded Dosing Regimens of GW640385X/Ritonavir Compared to Open-label Current PI Therapy in HIV-1 Infected, Protease Inhibitor Experienced Adults for 2 Weeks With Long-term Assessment (>48 Weeks) of Safety, Pharmacokinetic and Antiviral Activity of Selected 385/RTV Dosing Regimen(s) vs. a Ritonavir-boosted, Protease Inhibitor Containing Regimen

This is a two phase study (randomised and non-randomised phase). The randomised phase will initially examine 4 blinded doses of GW640385 boosted with rtv (with continuation of current background therapy) in comparison to an ongoing, open-labeled rtv-boosted protease inhibitor (PI) regimen for 15 days. At the Day 15 visit, all subjects will optimize background therapy. Additionally, subjects receiving the lowest dose of GW640385 will be re-randomised to one of the higher doses and subjects in the control arm will receive a new rtv-boosted PI based on resistance testing at screening. Subjects will remain in the randomized phase on one of these 4 continuing treatment arms for at least 48 weeks. An interim analysis will occur during the randomised phase to select for a dose of GW640385 to evaluate further in Phase III studies. After dose selection subjects will move to the non-randomised phase of the study. In the non-randomised phase subjects who are receiving GW640385 will be assigned to final selected dose for assessment of long term safety, tolerability, pharmacokinetics, and antiviral activity.