Keypressed and crt form!!

Hi all,
I want to have a loop like such :
repeat
  get_byte; {this gets a byte from the serial port, using a pre-made unit}
until keypressed;

I want to be able to press the buttons and use the mouse at the same time
that the loop is running. Keypressed seems to work, but a crt form appears
on the screen, and I have to minimize it to get the effect I'm looking for.
I can then use the mouse to press buttons, while the loop is in the cycle.
Is there a better way than using keypressed?
Thanks
Ray