I should have mentioned that I tried Excel files but had the same result, makes me think it's a good old Microsoft problem...any other suggestions on which files I could try opening without having to rely on Microsoft formats?
p.s. I attached the error messages, error1 is the first error message and after I click OK I get error2
Microsoft has a support article on this error message: http://support.microsoft.com/kb/839428
You may also find it helpful to update the driver for Excel files that Tableau uses. You can install a version from http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=23734
the above fix didn't work, but it prompted me to look for solutions on the web and found this page, and after applying all the fixes suggested, it now works