We will need more information to be able to help you:

  • Which OS are you using? Is it up to date?
  • What avast! version and VPS file (virus database) number?
  • Which actions have you taken to try solving the problem?
  • Do you use a firewall? Which one?
  • Do you have any other antivirus installed in your system?
  • Any other security programs that could interfere?
  • Any useful information into Control Panel > Administrative Tools > Events, specially ‘Errors’.

To know how to post a screenshot, see http://forum.avast.com/index.php?topic=8982.0
You can use Gadwin PrintScreen to get a screenshot (http://www.gadwin.com/printscreen/) or the free version of WinSnap 1.1.10 (http://www.filehippo.com/download_winsnap/?2173).

I suggest you search this board for Yahoo messenger. There should be more users with the same trouble…