Excel files only open as read-only

One workstation with Win XP Pro SP1 and another with Win 98 (2nd Ed) can only open some Excel 2000 files stored on the NT4 network server as read-only if they were created on another workstation. No problem in opening similar files saved on the server by that workstation. Does not apply to all Excel files created on other workstations, but both workstations have the problem with the same files. Other workstations using Win XP Pro, Win 98 or Win 2000 do not have the problem. All users are set up with the same rights and no files are saved as read-only or have read-only attribute. No similar problem with Word 2000 files. Any suggestions to track down this problem will be much appreciated.