Are you running Excel 2010?
Then it is likely that the security settings from Excel prevent you from opening these files.
By default, Excel 2010 only opens xls files that have formatted according to the Office 2003 document structure and to the Office 2010 document structure. It also depends if the files are stored in locations that Excel 2010 regards as unsafe (practically your complete system is deemed unsafe). Did these files come from LAN or the internet? Excel 2010 tagged them as such and will not open them either.
Excel 2010, you just have to love it. Anyway, Open Excel 2010 and select the ´File´ tab. In there is an option called ´Trust settings´. Over there you can relax the very strict security rules that are employed by Excel 2010.
If the need arises, I can put some PowerShell scripts up for download here that use Excel 2010 to convert Excel documents that use a structure not deemed safe anymore by Excel 2010 to an Excel document structure (or CSV) that Excel 2010 does not have a problem with. You should not have to go through the same misery as I did with a security consious Excel (or complete Office 2010 for that matter).