You seem to be infected... Do you use a firewall? Which one?
Perhaps not if it was the Network Shield that detected the exploit, but we need more information to be certain.
What was the file name, where was it found example (C:\windows\system32\infected-file-name.xxx) Or perhaps it was an internet location (URL) ?
Check the avast Log Viewer, Warning section.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall avast - uninstall, reboot, install the latest version, reboot.