From your question I guess you are trying to fix this in the Windows (not Excel) Read Only attribute in the file's properties.
If that is true, then I would take a look at the security options within Excel.
In Excel 2003, you will find this under Tools / Options on the Security tab where you specify the password behavior
In Excel 2007 this has been moved to Office Button / Save As / Tools / General Options
There you might find the sheet has been saved with a password that lets you open file the file as read only without a password, but requires a password to modify it. The Open Dialog box could create the impression that you are dealing with a Read Only file.
If you do not know the password to open the file for update, the only way I can suggest to get around the problem is cut and paste the the sheets into a new workbook.