If you feel secure and you are not afraid of viruses, there is probably no need to install some antivirus software with real-time protection on your computer. But from time to time, e.g. if you download something from the internet it's a good idea to scan it before you run it.
That's exactly the time for always updated free online antivirus scanning services. There are many of them, some can check the whole computer and some are only for checking selected files, so choose from the updated list ...
And if you want, check a list of a free antivirus and anti-malware tools.