e.g. someone with broadband as me on dial-up probably wouldn't get too far).
Well, if the OP doesn't have good TV reception where they live, I doubt that they are on broadband, but that's just a guess.
As DavidR suggests, please post a sample link to something that does not work anymore after installing avast, and we'll check it out (I have comcast now, but I have FIOS being installed on friday, finally!)
It's a good chance that a previously installed A/V program is causing the problem as well. Please let us know, and we'll go from there.