Critical messages

Can somebody help, please: I keep getting messages from Avast telling me that my computer is in a critical situation yet, when I do scan it reports everything is OK.

What version of avast?

What OS ?

A screenshot of the message would help.

when you open avast and look at the gui… does it say that some part of avast is turned off ?

if so, and you are not able to turn it on, try avast repair
control panel > ad/remove programs > avast > uninstall/change > repair option …wait a minute and reboot

and if this is a new avast install… did you remove your previous used AV before installing avast?
General: Uninstalling a third-party antivirus software. http://www.avast.com/en-eu/faq.php?article=AVKB11

http://www.screencast-o-matic.com/screenshots/u/Lh/1392038576647-8291.png

A big thank you to all of you who replied to my query. I shall try your solutions. Thanks again,