Greetings Uri!!
this line that blows in the debugger : 'SET LIBRARY TO "PRINTSCR.FLL" bothers me a bit.
If you are pathed properly on your development workstations - you won't see any errors -
try hardcoding the path of the library file in your 'set library' command -
also -
isn't there something you have to do to 'register' the library file [similar to an ocx registration ?]

regards [Bill]
>Hi everybody!
>
>I wrote an .fll that copies an area on the screen to a .BMP file.
>On several computers I reseive an error message
>"Library "c:\triwin\printscr.fll" is invalid.". On every computer
>in my office there's no problem. I saw in the debugger that it happens during 'SET LIBRARY TO "PRINTSCR.FLL"' command.
>
>TIA.
>
>Uri.