A 35-year-old HIV-positive man comes to you with a 6-month history of progressive memory loss and incontinence. He is taking zidovudine and a protease inhibitor. Examination demonstrates deficits in cognitive and fine motor control functions. Lab show a CD4 cell count of 25/mm3. MRI studies reveal moderate brain atrophy but no focal lesions. LP shows no CSF abnormalities. Most likely diagnosis?