Hi,
Here i would like know legal side of ‘Avast Free Antivirus 8’
Is this Legal to install and use ‘Avast Free Antivirus 8’ within company premises?
No.
avast! Pro is so inexpensive, it’s easy to be lega!
You can see more info here: http://www.avast.com/en-eu/business 
Not only that but its cost would be a legitimate business expense that can be set against tax, unless of course they aren’t paying tax either. Though the penalties for that (tax evasion) can be more severe.
Microsoft Security Essentials is the only free antivirus available for small businesses (up to 10 PCs, I think) but the protection is inferior to that offered by Avast and it’s more heavy on resources.