You can either:
1. Double click the avast4workstation-x.x.x-x.i586.rpm
file which should launch your package manager.
2. Go to the location where you have saved the file, press F4
this will open a console window, type: su
press enter then input your root
password and press enter. Now type: rpm -ivh avast4workstation-x.x.x-x.i586.rpm
then press enter.