kelmaz... the error you're getting indicates that you don't have the font used installed on your system. if you know what font was used in the presentation, you should just be able to re-install it. note that if you have not tried opening the presentation from within an administrator account, it would be good to do so.
are you using powerpoint 2003 or what version? i know in some versions, you can go up to Format and choose Replace fonts. then you should be able to open it and edit it.