In the Inventor 2012 System Requirements, we can read this concerning Excel:
Microsoft® Excel 2003 through 2010 for iFeatures, iParts, iAssemblies, thread customization, and spreadsheet-driven designs
There is however one exception to this system requirements paragraph that is not mentioned explicitly and we just found out about this exception via one of our beta testers: the free Microsoft product called "Microsoft Excel Starter" cannot be used with Inventor as a replacement for the full Microsoft Excel (which is of course not free).
The explanation is relatively simple.
Microsoft Excel Starter comes with the OEM version of Microsoft Office 2010 (named Office Starter 2010) and gets installed as a "virtual application".
Because of this, the app does not register itself in the Windows registry.
And this means that Inventor cannot find Microsoft Excel Starter via the Windows registry and thus iParts/iAssemblies, iFeatures and threads will not function properly with Inventor.
You will have to upgrade to the full Excel product to make things work.
Conclusion: there is no free iLunch.