Well, maybe not, the version you linked is a licensed version, so I think it would be ok for you to install it. You'd have to check that with the company, and of course, the PC owner, too.
The free version of the AVG suite is somewhat crippled by comparison.
I believe it's quite OK for a computer repairman to install a free AV on a customers' machine, subject to the customers approval, and that only the time spent installing it is charged for.