I have the exact same problem. Worked OK at first, then I had to update .NET framework for other programs to run, now MathCAD wont run. From Add/Reomve programs I have the following versions of the .NET framework. 2.0 Servicepack 2 3.0 Servicepack 2 3.5 SP1 Dont want to uninstall servicepacks and potentially ceate new problems elsewhere.