.XLS files are the previous version's format. Even an old copy of Excel will open and manipulate those files. Excel 2007 uses the newer .XLSX file format.
Sorry if I geeked-out on you.
Ed
I do not even have an old copy of Excel. I do have Word though. I use Adobe Acrobat Reader, and Open Office. I had to recently change my hard drive, so I haven't re-downloaded Open Office yet.