I just went to their forum and saw complaints about being bσɱbarded with invitations to upgrade. I recall having trouble removing it from my laptop. I ended up just using Linux with Firefox and that worked well. But this won't be possible for my elderly father.
This is really the only thing "wrong" with AVG. It does the job it's supposed to do just fine. But it's open source (free) so of course they'll "bug" you to pay for what you're using, occasionally.
This can be a little difficult for older generations to "understand," but if your dad can swing it, AVG is the way to go. The alternative is spending $100+ on McAfee or Norton, both of which will be utter resource hogs and tell your dad that he has a virus every ten minutes.
IMO, the occasional pop-ups asking you to upgrade are more than worth it. After all, you're not paying anything for it :)
Avast! is another alternative to AVG. It's also open-source, and I'm not sure if it "bothers you" with pop-ups to purchase as much as AVG or not-- but again, if you're not paying, you'll be dealing with the pop-ups to some degree or another.