Windows File Properties Problem

We are using AutoCAD 2004 and Windows XP Pro. We are used to filling out information about each drawing using the DWGPROPS command. This information can then be accessed externally by right-clicking on any .DWG file in Windows Explore and selecting "Properties". The "Summary" tab of the Properties dialog box is especially useful in identifying drawing contents without having to actually open the drawings with AutoCAD. Unfortunately, we have some .TIF files which are attached as XFEFs. For some strange reason, we can't seem to SAVE any information which is typed into the Summary area using Windows File Properties (in Windows Explore). We can type in the information, but as soon as we click on the APPLY button, we get an error message that says "Changes to Summary properties could not be saved; access has been denied. The file may be marked read-only or may be in use by another application." I don't understand why this is happening because the files are NOT read-only, and no person or application is using them when we try to type in the summary info (using Windows Explore). We have no trouble doing this with .JPG files, but are unable to save any file Summary properties info for our .TIF files. Has anyone else had this problem or know the solution? We have many different software packages for viewing, editing and saving .TIF files (such as Photoshop). We've tried saving copies of the .TIF files, and renaming them. But we always get the same error messages. I don't think this is a "permissions" issue because I have Administrator privileges and don't have any trouble typing and saving Summary properties info for file types other than .TIF. Any solutions would be greatly appreciated.