Microsoft release update "KB976264" that adds better compatibility for older games.
Good news it that one of this game is Warcraft 3.
So if You are having problems with Warcraft 3 on windows Vista/7 You should install this update.
You can download update via Windows Update.
Or download it manually:
Here You can find list of application/games that have better compatibility when You install this update.
This update also increase compatibility with Diablo.
If You have more problems with Warcraft 3 on Windows 7 read this post.