Which version of Autodesk Inventor* are you using? It appears the 2009 version supports multi-core CPUs.
According to the Autodesk knowledge base, it's possible to Activate/Inactivate multiple processors. Have you attempted to change that setting?
This question is fairly specific to the Inventor application, so you should also send your question off to Autodesk. If they reply back, I would appreciate hearing their take on this issue.
Intel Customer Support
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