We recently got a case that came into Product Support in which Inventor 2013 was hanging at the splash screen on Windows 7/XP. The culprit was a Microsoft KB2633171. Once you uninstall the update, the software would function properly.
It was a bit of luck to solve the problem. Basically, we had to find out the last working day the Autodesk software ran and then we checked for programs and updates that were installed on that day.
If you google KB2633171, you will notice some interesting effects of this KB patch.
Thanks to Jeffry Asbury for sharing this tip with the community.