Have you fooled around with your extension sets and control panels yet? That usually solves the problem. Just take all non-game essential extensions and CPs out of their respective folders, put them in a 'Disabled' folder, restart, and see if the game works. If it doesn't, then you most likely have a system problem and will need to reinstall the system in order to play. If it does work, then start adding the extensions in and seeing if it works, until you hit the one which is causing the problem. It's pretty time consuming, but it's the system I use and it's treated me well up until now. If you have any more questions, please feel free to ask.
Blizzard.