Dissapearing Avast Antivirus

I had recently downloaded and installed a free version of avast on my PC having vista.
The avast remained for 3-4 days and after that it was not seen in the notification area.
It does not even appeared on the right click menu of any folder.
When I checked the installed softwares in control panel > programs then avast is not seen there.

I reinstalled the software, but it again dissapeared after 3-4 days.

Will you pls let me know what might be the reason behind this and hot resolve it?

strange… ???

what AV did you use before installing avast ?
have you uninstalled it ?

it is also recomended to run a removal tool to clear all leftovers

Uninstallers for Security Software
http://thewebatom.net/uninstallers/security-software/