You haven't installed WinDVD or any new sound/video codec packs, have you? They gave me trouble at first, but I decided to reformat later and it worked out again. Really, if it works one day and the next it doesn't, you need to backtrack the changes you've made to your system.

If you're absolutely certain you have not done any manual alterations to your system configuration, the game files may be damaged. If so, back up your save game files and reinstall the game.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram