Sorry me… I haven’t see you’re using the Professional version.
Besides the Virus Page of the Outlook/Exchange plugin settings (right click the Standard Shiel Task on Enhanced Interface and choose properties) you can set this options into avast4.ini file:

ShowVirusTag=0 at section [AAVM] to avoid the displaying virus advice (yellow/red layer that pops up when a virus is found). A reboot may be required for the changes to become effective. Didn’t you find this at my ‘Settings’ link?

More information here: http://forum.avast.com/index.php?board=1;action=post;threadid=9810