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.