Lamivudine or emtricitabine are commonly used in combination with other drugs for first-line treatment of HIV infection. Although effective initially, many people later on develop resistance to some or all of the drugs (including lamivudine) leading to virological failure. Resistance is associated with characteristic mutations, which for lamivudine is the M184V mutation. A change to new, active drugs must take place when patients fail their current regime, to regain control of the virus. Although there are other types of drugs available for second line treatment, there is currently no fully active, well tolerated cytidine analogue that can replace lamivudine in a second-line regimen when patients fail first line treatment. This study will measure the efficacy and safety of AVX754 (a novel cytidine analogue with activity against HIV resistant to other nucleosides) as part of a new regimen to treat patients who have failed treatment containing lamivudine, compared to the best alternative new regimen which continues to include lamivudine.