Anti-Virus protection that's free?

I just had a call from a company that told me I had a virus, and told me to go onto this specific website to get a free scan by them. When I went on the website under the instruction of the guy on the other end of the phone (who was really patronising) he told me to click on Run to the scanning process, he then became frustrated that I was asking questions about the company that made me think that it was a scam, so I put the phone down. Scared for my computer and my personal details, I wish to download an anti-virus software, and firewall precautions to protect my computer.