malware

about 6-8 months ago I downloaded an "adobe" alert that gave my Mac air a malware virus. I was able to delete the application, but somewhere on the book its still there because every 1st and 15th of the month it downloads again. the symptoms of the malware are they change my default search to yahoo... how can I search my laptop without downloading a malware product to find where its hiding on my device. Thanks in advance