Either I'm hvaind deja vu or it;s been mentioned before, I think it was driver troubles. Make sure you have the latest drivers installed and the newest October 2006 version of directx.
It could be a problem with your system fonts. Try reinstalling your fonts from your Windows CD.
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Had this once and it was caused by my gfx card which was overheating! So be sure to check the temperatures of your gfx card if you experience this problem again. In my case I just had to clean the fan on the gfx card since it was full of dust... Because : dust = lower fan rpm = higher temperature = not good