Free, Fast, Antivirus Software?

I have a friend with a cheap 4+ year old laptop, and lately she has been getting these weird viruses (most of the time in the form of “antivirus software”) and I (being the one that is good with computers) have to go through the hassle of trying to uninstall the stupid software.

She is not willing to spend money on buying antivirus software, so I am hoping to find a free antivirus software she can use, to at least keep the number of viruses to a minimum. It would need to be very fast, considering her computer is really slow as it is. She is not the kind of person that will remember to press “scan” or “update” and so would need to do a lot automatically.

I am looking for a free antivirus software, that will not slow down a 4+ year old laptop, and that will do a lot automatically. I know I am kind of asking a lot, but that is why I am asking, I am trying to find out if it exists at all.

Thank you for all your help,