Log into your web browser--Safari, Firefox or Internet Explorer are probably the best to use.
Go to http://abc.go.com/player/ which is ABC's online site that shows all episodes of their shows for free. You can go to other sites like Hulu, but it isn't necessary because ABC offers its own exclusive player and content.
Download the ABC Full Episode Player, which you can do by clicking on the left side of the screen. A window will pop up, you will have to click the terms and conditions and then "allow" it to download. It's for both Mac and PC users.
Let the player take you to the menu for your favorite ABC shows. There you can watch the most recent full episodes of each show and most of the older ones.
With an S-Video Cable, you can also run episodes of the shows from your computer to your television, as long as it has an S-Video input. There is a bit more setting up on each type of computer and other ways to run audio so it sounds better.