Virus scan runs automatically once a week. I also run Ad-Aware and/or SpyBot at least once a week, sometimes more often if I feel I've been hit - believe me you can feel it when a website is installing trackers - lots of disk action when you're not doing anything. Also if I've wandered onto some sketchy websites (I'm not talking porn, but some off-the-beaten track shopping sites can be strange) I always clean afterwards - no telling what those strange sites put on your computer.

Once a month I defrag and/or run disk cleanup..