Aside from the fact that you are replying to a post that is close to 6 months old, you are mistaken regarding the file extension issue.
You will note that the OP said "...when i tried to open several files in Excel 2007..."
MS introduced the XML file format (xlsx) when Office 2007 came out. In addition, earlier versions of Excel can open xlsx files if the Compatibility Pack is installed:
Since the OP said he is using Excel 2007, the file extension of xlsx is probably not the root cause of the problem. It's too bad that he never came back and told us what the actual problem was.
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message edited by DerbyDad03