Cannot Download/Install Avast free antivirus?

Avast Free Antivirus / Premium Security
1

Have tried to install Avast several times but cannot do --Box says something like “Fatal Error. Avast does not trust installer, etc. Try again”. There is nothing on the “log” that tells me the reason or problem. I have no other antivirus program running, etc.
Have Windows 7 Home Premium, SP1, with plenty of space for the download. Any suggestions as to what might be causing the problem?
It’s working fine on my wife’s laptop, by the way.

Thanks. 5/16/14

2

what AV did you have before avast?
did you remove it according to vendors instructions / removal tool ?

General: Uninstalling a third-party antivirus software http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle

3

Step by step instructions on successfully installing avast!:
http://goo.gl/VLXde