Unable to install Avast PRO antivirus

My Mam is having problems trying to install this programme.
Dell had worked on her computer before and installed System Mechanic Anti virus suite, as this was , well to be fair, rubbish, she uninstalled it. Then tried to put the free version of Avast back on her computer. This she was unable to do as she could not register, user name (email address) already registered So she decided to pay for the Pro version. The email she gets to start the installation process has an attachment, windows can not open this as it does not recognise it and so she cant proceed.
As she is unwilling to use her computer on the net at the moment and I have know access to computer, this is a bit of a second hand conversation, also my mothers computer knowledge/ ability is quite limited so some of what she has told me might be a miss interpreted, but any help/ thoughts would be appreciated.
Its a Dell Inspiron laptop running windows 7

The registration licence is slightly different for Pro and up… Save the licence file to the desktop
Install Avast
After the reboot double click the licence and it will install