set your control box to false
that removes the buttons
then create your own command buttons
mim, max, exit
in the exit put your code...that way you can act on it
accordingly...shut down if they say yes...
simply resume if not....

PS..I personally would never give the option of cancelling
out of an install as it could have serious consequences regarding your registery if you quit in the middle of loading files...especially ini' etc.

Have fun...