Drug resistance testing can be used to see which anti-HIV drugs are likely to suppress the growth of HIV and to select an anti-HIV regimen for HIV infected patients who have failed previous drug regimens. Therapeutic drug monitoring (TDM) is a process that involves measuring blood levels of a drug and may further increase the benefits that resistance testing offers by optimizing protease inhibitor (PI) drug concentrations. The purpose of this study is to determine whether changing the dose of PIs, as indicated by TDM, reduces the viral load in PI-experienced patients.