Excel 2007 : Excel 2007 cannot open XLS on Vista

I have some Excel 2003 .xls files that I can't seem to open in Excel 2007 on Vista OS. All XP machines with Excel 2007 open the .xls files in question just fine.

"Excel cannot open the file 'filename.xls' because the file format or file extension is not valid. Verify the file has not been corrupted and that the file extension matches the format of the file."

I've searched, but have only found reference to backwards compatibility. Saving 2007 for open in 2003.

Any thoughts?

Thanks,
Jamie