Best free anti virus for a laptop

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Lately, My laptop shuts down for reasons I do not know. Advices were that it overheats. It does. Also they say that might be having viruses. This laptop was handed down to me and they informed me that there were no virus set up in it. But when I download anti virus softwares from the internet, my laptop says its incompatible with my laptop. My laptop is Acer TravelMAte 3275WXMi. What possibly could be done. Any responses would be appreciated so much. Thank you guys!