.1091 Def update error (not enough storage)

[dagrev]

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.

[dagrev]

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.