Obviously, you can try to update the avast database manually to see if it fixes the warning.. If not, you might have to reset Windows Security Center. I know how to do it in Vista because I had to do it before.. if your running Vista you can try this..
Type "Computer Management" in the start menu search box.
Click on "Services and Applications" on the left hand side
Click on "Services"
Stop the Windows Management Instrumentation service
Delete the C:\Windows\System32\wbem\repository folder
Restart the Windows Management Instrumentation service
Reboot your computer
This should reset Windows Security Center and fix the problem.. but only if your running Vista.. I'm sure it's similiar in XP but you might want to make sure before you mess with folders in your windows directory.
Hopefully that works.
Here's where I got the info..
http://www.pchell.com/support/multiple_antivirus_in_security_center.shtml~spiRits3033