Problem with an excel file generated with a PHP class

Hi all, I am getting this error when generating an excel file blah.xls cannot be accessed. The file may be read-only, or you may be trying to access a read-only location. Or, the server the document is stored on may not be responding. I get this error if I click 'Open' in the dialog box, or if I save it and then try to open it. At this point I am rather tired and may be over-looking something. Can someone shed some light on the problem for me? Thanks!