To do it legally, buy a new retail Tiger DVD.
Alternately, you may be able to connect to
your powerbook from the mac mini in target
disk mode and boot to the install disk on your
mini and choose the powerbook harddrive as
the install disk - this may or may not work.
Also - any data on the powerbook that you do
not have backed up will be lost if this method
does not allow archive and install.