This study is a 48-week study to evaluate the efficacy and safety of an investigational regimen combining FDA approved HIV drugs in antiretroviral-experienced subjects failing on their first highly active antiretroviral therapy regimen.

Inclusion Criteria:

• Currently on an initial HAART (highly active antiretroviral therapy) regimen of 3TC/ZDV or 3TC/d4T and a PI (protease inhibitor) -boosted or unboosted- or NNRTI (non-nucleoside reverse transcriptase inhibitor).
• Plasma HIV - 1 RNA was <400 copies/ml on at least 2 documented occasions prior to viral rebound.
• Have a plasma HIV - 1 RNA value >400 copies/ml and <10,000 copies/ml on 2 documented successive occasions (including screen) separated by at least 2 weeks.
• A CD4+ lymphocyte count less than or equal to 100.

Exclusion Criteria:

• Have not taken Abacavir (ZIAGEN or TRIZIVIR) or tenofovir (VIREAD) previously.
• Have not had an AIDS defining illness within 30 days of screen.
• Pregnant or breast-feeding.
• Specified viral genotypes upon screening.
• And other inclusion or exclusion criteria to be evaluated by the physician.