It seems that the C:\Documents and Settings\<username>\Application Data\Microsoft\VisualStudio\9.0\Windows.prf gets corrupted from time to time, causing errors such as:
- FeatureModelDSL reports a Package Load Failure when Visual Studio starts.
- Opening a .fm file gives an "Unspecified Error".
- Right-clicking a feature model and selecting "Show Confeaturator" gives the error message "It was not possible to load Confeaturator: Value does not fall within the expected range".
Deleting the file and letting VS to regenerate it restores everything back to normal. However, it should be investigated why that is happening.