PowerPoint saves files as "read-only" only

Hi, Everyone:

I use "PowerPoint 2008 for Mac Version 12.1.1" on "Mac OS X Version 10.6.8".. And now I have a problem.

Each time I attempt to save a PPT file, it says the file is "read-only" and you need to select a different file name to save it. However, when I open the newly saved file later, it is yet another "read-only" file, and if I change anything, I have to give yet another new file name to save it.

If I go to the menu File -> Properties, I see that the box for "read-only" is checked, and it is not active to be unchecked.

I have been using Mac OS X and PowerPoint for a long time, and have never had this problem before.

Could anyone tell how to resolve the problem?

Thank you for reading and replying!