Although “nipun kumari” double posted, just for future reference for other users that might have some similar problem, please read the following post:

http://forum.avast.com/index.php?topic=84487.msg691455#msg691455

Usually, booting into Windows Safe Mode with the normal user would be enough to remove Avast with http://www.avast.com/uninstall-utility so you can re-install it again under Windows Normal Mode, using the latest stable version, downloaded ANEW.

For those who tried but couldn’t, read the above linked post and try to follow those instructions.

HTH.