tkivle,
I assume you're talking about the license key. Based on the replies to other posts about this problem: Usually, the problem is a typo when entering it. Cut the key from the e-mail you received, and paste it into the dialog box rather than typing it. If that doesn't work, or if that is what you did, other things to try are re-registering to get another key and reinstalling avast!.
Regards,
Hornus