Legality of "free" software?

i am running a small business fixing and optimising PCs via Remote Desktop software. I was wondering if it is legal to install a free virus scanner such as AVG free edition onto some-ones PC using remote desktop? I heard that I may run into legal issues doing this as the "free" isn't always as free as we might think. Does anyone have any advice or a decent virus scanner that would allow me to do this legally?