Hm, I'm not sure why that's happening, since I'm on Windows 7, also. Perhaps someone more knowledgeable about Game Maker 8.0 knows the cause (I'm using Chevy Ray's screen scaling script)?
Are you going to do a HD remake?
Alastair, you little imp, you. But if you're being at all serious, no, I don't have any plans to do an HD version.
The hitting felt right, and pretty forgiving in a good way. I thought the NPC's AI felt really well tuned and challenging, I haven't beaten him yet but I won a couple games.
...
Maybe you could make the NPC's clothes and gear a different color so you can recognize your character quicker?
Glad you like the game, Ted! I'll probably add another character before I release another build, so you should have no trouble recognizing who's who then.

Are you planning to add gamepad support, too?

Definitely!
Something still bothers me : The windows screen seems to small, when I'm on the upper area, I can't even see my player when the opponent serves. This is not a big deal because it only lasts one second or two but...still annoying.
It reminded me Mario Tennis of course, a game I love, and I hope you'll push this forward!
Yeah, it's a bit weird to have the players cut off at the top. I'll see what I can do about that without changing the screen resolution (for some reason I'm a bit hesitant about that).
I'm having a lot of fun working on this game so I'll keep pushing at it. Thanks for you feedback!