I may be wrong, but it's my understanding that print screen was not included in dos commands after DOS 6.22 (ie.. win95 & up). It came up a couple of years ago from some of my customers running dos programs like yours and wanted this feature. I don't know the program or a source for it, but they solved the problem by purchasing a cheap program that supplemented the dos commands in windows 95 and included the print screen command.

I haven't tried it, but you might be able to copy the print command from dos 6.22 into the Windows\Command\ directory and see if it works.

Good luck.