hello everybody

when i try to open the some .ppt files but just some files, i get the error code "PowerPoint can't open x.ppt".

in 'help' it says:
PowerPoint can't read this file
The file you tried to open may not be a Microsoft Office PowerPoint file (.ppt). Check the original file to determine the file type. You may be able to edit the file in a program designed to use that file type.

but it's a .ppt file and my friend is able to open this file but i can't. does anyone know what is the problem with me? thanx.