I had this problem today. Turns out that I managed to delete the very first support file path in my list in the Options - Files Tab... That path being:
C:\program files\autodesk\acade 2010\acade.
In my hurry to add a new path for some new functionality I created, I managed to fat-finger my list.
Hope that is the problem you have also. Good luck. I know it really messed up my day not having my software working for a while there.
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