Are you getting anything (error/warnings) displayed to the screen ?
Are the VPS signatures getting updated, outside of these errors (right click the avast 'a' icon, select about avast), what is the VPS version (latest is 081001-0) ?
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow.