A few things to say..
The graphics could do with some improvement but that will come with time. The concept itself was simple, I mean it was pong, it could do with some work but overall it was a good effort in my opinion. I have made a pong remake recently and it had a few other features. Check it out if you want, its nothing amazing but it might give you some ideas.
A couple of things are that the computer paddle jerked about a lot, I think if you use a higher frame rate and a smaller movement increment it will be smoother. Also maybe allow the computer to have some leeway with the ball, say only move if the ball is +/- 5 pixels from its y value. And finally you need to limit how far the player paddle can move vertically as at the moment it is able to move past the top and bottom of the window.
Keep on with the flash! :)