Arm 1 receives memantine plus concurrent antiretroviral therapy; Arm 2 receives placebo plus concurrent antiretroviral therapy. The initial dose of memantine is increased each week for the next 3 weeks, unless a maximum tolerated dose is reached. Following a 16-week evaluation period there is a 4-week washout prior to re-assessment. After the washout period, all patients, including those formerly randomized to the placebo arm, are given the opportunity to receive an open-label, 12-week administration of memantine. [AS PER AMENDMENT 4/6/99: The open-label phase is extended by 48 weeks to a total of 60 weeks of therapy. Patients who did not receive the initial 12 weeks of open-label treatment are eligible still for the 48-week open-label treatment phase.]