I've had the same issue in the past. It was a new version of Microsoft Office that was the culprit. It seems to want to argue with Inventor. Something about how Office runs VBA in it's own little environment or something like that. Look for VBA7.msi in your Inventor installation media (You may have to download it from Autodesk, but not follow through with installation again, because I think the default behavior after installing Inventor is to delete the installation files). Install that and Inventor should run fine (for now). I've heard opening an Office application will undo your progress and you'll have to reinstall the VBA7.msi every time, but I can't confirm that. Apparently it's a known issue and Autodesk and Microsoft are working together on a solution. In the mean time, the customer loses.
Good luck