Running Office 2010.
When saving a document in Excel, a window pops up with the document name, followed by "class not registered".
And I can't save the document, any document.
Try running Excel with the /regserver switch. With Excel closed, go to the Windows Menu and choose Run. There, enter the following, including the quotes:
Of course, your path the Excel.exe may be different. This will start Excel and cause it to rewrite its registry keys and file association and the other plumbing that makes Excel work.