Can anybody help me with the following:

- from where can I download avg-free antivirus software for Linux?
- can you give me the complete set of installation instructions for Ubuntu?

Thanks in advance.

You don't need an anti-virus. Every program that try modify your system into "important" parts, like a virus or administrative programs can't do without your permission. Thus virus are useless.

Install Ubuntu is pretty easy, the instructions when you are installing are very clearly and friendly. But official instructions are here https://help.ubuntu.com/community/Installation. Search is not hard!

To answer your question, you can download AVG antivirus 7.5 for linux from here (see the link near the bottom of the page). Among others it comes in the form of a .deb package for Debian/Ubuntu, so you have simply to download it and issue the command
A User's Guide is also available (see the Dcoumentation tab in the same page where you download the installation file).

Anyway, I agree with pslacerda: maybe you don't really need an antivirus software for Linux. Looks for documentation about Linux security (or search topics here at LQ) and you will find out how to harden your linux with a good setting of firewall rules and eventually running a rootkit hunter and a filesystem integrity check tool.

Unless you always login as root, and regarding some posts on this forum, this happens more often then we think.
People are lazy, they don't like to type passwords all the time.