No antiretrovirals will be provided by this study. Participants will be followed for a maximum of 48 weeks. Participants failing at least one combination antiretroviral regimen will have a screening drug resistance test performed while remaining on the failing regimen. In Step 1, participants will begin a salvage antiretroviral regimen within 7 days of study entry selected by their clinician using results of the resistance test. Two weeks after initiation of the salvage regimen, participants will have timed plasma samples obtained for PI trough levels. The results of the trough level tests will be used to calculate a normalized inhibitory quotient (NIQ) in order to determine eligibility for randomization into Step 2 at Week 4. Electrocardiograms (EKGs) and trough levels will be performed at Weeks 2, 6, and 10; support interviews to promote adherence will also be conducted by the study nurse or clinician at these times. Some participants taking tipranavir may have additional blood collection at Week 2.