This page, part of Ubuntu Documentation, has some pretty good instructions on using it. It also tells how to install it. I hate to plagiarize what was written there, but this is what it has to say:
Installing AVG Free
This works for both 32 and 64 bit versions of Ubuntu 10.10 and above.
Download the linux version (.deb file): http://free.avg.com/download.prd-alf
Double-click on the file which can be found in your download folder then click on the Software Centre Installation button.
Reboot the PC to initialize AVG. Alternatively you could run
sudo /etc/init.d/avgd start
Using AVG Free
Open a terminal (Applications-Accessories-Terminal). Now update the virus definitions (while connected to the web) with
To perform a virus check enter
For example you want to scan your USB device:
First type on the terminal:
After you do this you will see a list of the devices with some extra information Look for the "UUID" number, also if the device appear to have a "LABEL" then use this last one.
Ok now that you know the "UUID" or "LABEL" if is the case
you can proceed with the following code supposing the "UUID" is AE45, Remember to copy the whole code even if it contains "-" ok now type the following:
sudo avgscan /media/AE45
Now the scan operations will be performed.