This study will be carried out as a Phase 4, Multi-Centre Open Label, Investigator Led, Randomised Pilot Study to compare the efficacy, safety and tolerability of raltegravir (Isentress™) with protease inhibitor-based therapy atazanavir/ritonavir (Reyataz™/Norvir™) in treatment naïve, HIV/Hepatitis C co-infected injecting drug users receiving methadone. Patients who are attending a drug treatment clinic, who are on methadone maintenance therapy, who fulfill standard criteria to receive antiretroviral therapy (ART) and who meet all other study specific eligibility criteria will be invited to participate in the study.