Easiest way is probably right click the slide > background. Choose white and
tick omit background graphics from master then apply( NOT apply to all!). You
may also want to apply a blank layout from slide show > layout > click the
blank one

Greg,
Just a thought, but have you tried going to Format - Background and checking
the box at the bottom of the dialog, where it says omit graphics from
background or something to that effect.
Also if you have PPT 2002/2003, you can create a second master with no
background and apply that to the two slides. Give the second master a name
which will help you recognize it in the Slide Design task pane.