mastana20,
Your post probably hasn't received an answer because it lacks some basic information.
What Operating system do you have ?
What version of Avast have you installed? You say you have un-installed and installed it several times - did you do this from the original download of Avast, or have you tried downloading another copy?
Did you have another antivirus program installed before Avast, if so what was it, and how did you go about removing it?
Can you post back some details.