Sounds like something is corrupt with one of the files in your User library (~/Library). I would try first with the Excel files in ~Library/Preferences/Microsoft/. Just move them onto your Desktop and see if Excel opens a file. If it still fails, they are not the problem - put them back into the folder and try one of the other files. Are the other Office programs OK? If so, it's probably unrelated to the 'Office' files in that Microsoft folder. This sounds quite solvable though.