I guess you had Tennis Elbow 98 which was an hybrid Win95/Dos, and that doesn't work on Windows XP. (the 97 version was only DOS)
The gameplay and AI of Tennis Elbow 2006 isn't that much different from TE98, but there's a lot of little improvements and fixes in the game itself, and the World Tour has been completely redone with a lot of more features : qualifications, double, score simulation, and so on...
The graphics didn't change much as well, but there are 2 new courts : the indoor hard and the blue-green cement; and you can play in 1600x1200 resolution.
Most of the sounds FX have been redone with better ones.
So I hope you'll like this new version !