Participants will have blood drawn to learn what anti-HIV drugs the patient's virus is resistant to-that is, what drugs are no longer effective against the virus. This is determined by analyzing the virus's genotype (detailed genetic structure) and phenotype (response to exposure to anti-viral drugs). Based on these test results and the patient's prior medication history, a drug regimen tailored to the individual patient will be prescribed. It may include one or two nucleoside reverse transcriptase inhibitors, such as zidovudine, didanosine, lamuvidine, zalcitabine, stavudine), a non- nucleoside reverse transcriptase inhibitor such as nevirapine or efavirenz, and a protease inhibitor such as amprenavir, nelfinavir, saquinavir, ritonavir, or Kaletra (a combination of lopinavir and ritonavir).