Powerpoint 2000 Problem - cannot open file

HI
Having a problem opening a powerpoint presentation using
Powerpoint 2K with SR-1.....

Was working before (and not the first times its happened)
but now when trying to open the file, i get the error
message:

"Powerpoint couldnt open 'name and location of file'
because no converter for this file type is installed"

Any help is appreciated
Cheers