Here's some code to get you started (I'm typing this in.. so watch for
typos.. and sorry for the sloppy code):

{$M xxx,xxx,xxx}  (* Make sure to limit the memory you use! *)
uses dos;

const
  kbdint = $09;

var
  regs : registers;
  OldKbdVec : Pointer;

Procedure STI;
Inline($FB);

Procedure CLI;
Inline($FA);

Procedure CallOldInt(Sub:Pointer);
Begin
  Inline($9C/
         $FF/$5E/$06);
End;

Procedure KeyBoard(Flags,CS,IP,AX,BX,CX,DX,
                   SI,DI,DS,ES,BP : Word); Interrupt;
Begin
  CallOldInt(OldKbdVec);  (* Call the old interrupt to process the key
                             if you don't use it *)
  STI;
End;

BEGIN (* Main *)
  GetIntVec(KbdInt, OldKbdVec);
  SetIntVec(KbdInt, @Keyboard);
  Keep(0);
END.

Hope that helps..
later..

save interrupt vector value for int 9
change interrupt vector nine to point to my code
  my code:
    check for keypress, if so log, pass control back to
    previous address of int 9

if you're using a log file, you might also want to tag onto the disk
read/write interrupts so that you don't write keystrokes to a file and
make the little hard drive light blink when nothing should be happening.