Keyboard INT 9 handler (Handles multiple keypresses!) Problem

Hello....
I want to detect multiple keypresses in my game I'm programming.
So I got the source code for this but when I compile it I get the next error:
Linking KEYTEST.EXE:
Linker Error: Undefined symbol _Set_Old_Int9 in module KEYTEST.C
Linker Error: Undefined symbol _keys in module KEYTEST.C
Linker Error: Undefined symbol _Set_New_Int9 in module KEYTEST.C

can someone please help me! :confused:
check the attachments, it's the source...

I use Borland Turbo C++