Photos enlarging themselves when making slideshow.

I recently purchased the standard version and am having a problem. At
first it was freezing up, but that was caused by a case fan which
went out and was causing my machine to lock under high stress, like
making a slide show. Now, everything is working well as it was
during my trial period. However, I cannot get the photos to display
at their actual size, no matter what I try. I am using maximum
settings for my slideshows (1600x1200, very high quality) which is
working well on all pictures at my screen resolution of 1152x864. The
larger pictures are resizing themselves to my resolution, but the
smaller pictures are totally unclear and blurry because they are
trying to enlarge themselves to larger sizes than they are. They do
not enlarge themselves to my screen resolution but it seems they are
trying to double themselves in size. The ones I shoot with my Nikon at
a high Res are no problem, but the small ones, I include in some of
the shows that I have downloaded from the web are the problem. Ones
like 640x480. They make themselves bigger, therefore becoming blurry
and unclear dues to the lack of picture info in them or low
resolution. Can you tell me how I can stop this ? The enlarge images
to full screen size box is unchecked.