Have you tried a repair of avast. Add Remove programs, avast! Anti-Virus, Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If I remember rightly someone experienced this out of synch update info and a repair worked.
Also when you right click avast icon, select Start avast! Anti-virus, what VPS version and date are shown there?
You can also try to update the VPS by clicking VPS version and Date line from here as well.