Group A: 2 tabs of 18 mg Concerta daily for Dementia

This study is being done to see if a drug called long acting methylphenidate (Concerta) is safe and effective as a treatment for problems with mental function in adults infected with HIV. A subset of patients with HIV-associated memory loss have a defect in the speed with which they learn and process information. Methylphenidate drugs, such as Ritalin or Concerta, have been shown on tests to improve the ability to rapidly absorb information; these tests are called "reaction time tests". These drugs are already FDA-approved to treat Attention Deficit Disorders: ADD or ADHD. At baseline, all subjects get tests of memory and brain function; then they are split into two groups. One group on this study will receive Concerta for 2 weeks, and a second group will receive a placebo x 2 weeks. After that period both groups will receive memory and other tests of brain function, and then the groups will switch. The first group will receive placebo and the second will receive Concerta x 2 weeks, followed by more memory and neurological tests. After that all subjects will have the option to receive Concerta for free for 8 more weeks. At the last visit all subjects get memory and brain tests again.