This study will look at people who have been taking anti-HIV drugs but still have detectable levels of HIV. The purpose of the study is to find out what happens in those people who change anti-HIV drugs when their viral load reaches 200 copies compared to those who change anti-HIV drugs when their viral load reaches 10,000 copies. This study will also look at drug resistance (how well HIV responds to drugs), viral fitness (how well drug-resistant HIV copies itself), and immunologic reconstitution (how well the immune system recognizes various infections, including HIV).