.1091 Def update error (not enough storage)

I’m getting an error when I try to up date the virus def. “Not enough storage is available to process this command.”

There is plenty of space. Prior to this I was using FF, avast was updating and I had a flicker and the screen went black with some kind of error message for about one second then it rebooted. Check disk could not run (no access to the volume I believe it said) then it started.

I’m going to reboot now to see what happens.

Upon reboot chkdsk ran and corrected about three screens of things (more than I’ve ever seen), and booted normally. Avast updated itself and is working fine.

I had not been adding things or tinkering so I’m confused as to what was gong wrong and why chkdsk needed fix things. I could rollback but have no idea when the problems began to happen.