A Phase IV Randomized, Multicenter, Open-Label Study to Compare the Safety, Tolerability and Efficacy of Trizivir (abacavir 300mg, lamivudine 150mg, and zidovudine 300mg) BID vs Combivir (lamivudine 150mg and zidovudine 300mg) BID plus atazanavir 400mg QD in Antiretroviral Naive HIV-1 Infected Subjects over 48 Weeks