I have this same problem, Computer is HP XE-763, using Windows ME. I tried to load the link given, but can't get it, tried the do net detect, but still won't get it.
Do you use any other antivirus in your computer?
Can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
You must be connected to the internet while repairing.