Does avast tell you it is up to date when you do a manual update check ?
If so:
Try a repair of avast:
XP - Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select ‘avast! Anti-Virus,’ click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.
This has in the past resolved this out of sync issue between reported and actual VPS version.