There are really two print screen functions: 1) print current screen
 snapshot, triggered by PrintScreen or Shift- PrtSc or Shift-gray*, and
 2) turn on continuous screen echo, started and stopped by Ctrl-P or
 Ctrl-PrtSc.

 1) Screen snapshot to printer:

 The BIOS uses INT 5 for this. Fortunately, you don't need to mess with
 that interrupt handler. The standard handler, in BIOS versions dated
 December 1982 or later, uses a byte at 0040:0100 (= 0000:0500) to
 determine whether a print screen is currently in progress. If it is,
 pressing PrintScreen again is ignored. So to disable the screen
 snapshot, all you have to do is write a 1 to that byte. When the user
 presses PrintScreen, the BIOS will think that a print screen is already
 in progress and will ignore the user's keypress. You can re-enable
 PrintScreen by zeroing the same byte.

 Here's some simple code:

   void prtsc_allow(int allow) /* 0=disable, nonzero=enable */
   {
     unsigned char far* flag = (unsigned char far*)0x00400100UL;
     *flag = (unsigned char)!allow;
   }

 2) Continuous echo of screen to printer:

 If ANSI.SYS is loaded, you can easily disable the continuous echo of
 screen to printer (Ctrl-P or Ctrl- PrtSc). Just redefine the keys by
 "printing" strings like these to the screen (BASIC print, C printf(),
 Pascal Write statements, or ECHO command in batch files), where <27>
 stands for the Escape character, ASCII 27:

   <27>[0;114;"Ctrl-PrtSc disabled"p
   <27>[16;"^P"p

 If you haven't installed ANSI.SYS, I can't offer an easy way to disable
 the echo-screen-to-printer function.

 Actually, you might not need to disable Ctrl-P and Ctrl- PrtSc. If your
 only concern is not locking up your machine, when you see the "Abort,
 Retry, Ignore, Fail" prompt just press Ctrl-P again and then press I. As
 an alternative, install one of the many print spoolers that intercept
 printer-status queries and always return "Printer ready".