A 68-year-old man who was a diagnosed case of HIV infection for last 12 years was brought to outpatient depament for evaluation. His wife repos that for last 6 months she has been noticing changes in his behaviour. She fuher added that 'He appears uninterested and doesn't seem to care about anything. I haven't seen him crying or saying something negative, but may be he is getting depressed. He is also having difficulty remembering things ' The patient has been irregular with his HIV medications. His last CD4+ cell count, measured 5 months ago, was 210/mm3. While walking , patient was observed to have subtle jerking movements. Which of the following is the likely diagnosis?