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I have just removed Office 2007 Professional and installed Office 2010 Professional Plus RTM on my Windows 7 Ultimate x64 machine.  I chose the x64 bit version of Office.

However, all my .xls and .xlsx file associations were missing.  And it did not help going to File -> Properties -> Change association as no matter what I did, I could not add Excel to the list of applications.  No amount of Repair / etc worked.

So I fired up Regedit.exe, went to HKCR\.xlsx and deleted that key.  Instantly the .xlsx files on my computer were associated with Excel.  I could now right click on .xls files, and change their default application to Excel.