A 55-year old man presented with a two-day history of headache, fever and generalized weakness. He had received a cadaveric kidney transplant 5 years earlier. His medications included 5 mg oftacrolimus twice a day and 10 mg of prednisone daily. On neurologic examination, he was confused and incoherent Cranial nerves were normal, but he had a hazy left retina. Magnetic resonance imaging of the brain with the administration of gadolinium showed multiple enhancing lesion in both cerebral hemispheres. The most probable diagnosis is -