Poor quality images on projector despite high quality scan

Hi,
I'm due to give a lecture on my experiences in Australia, and I need
to show some pictures I've taken in that regard. I've also downloaded
pictures of exotic, yet common animals in Australia, that I don't have
any pictures of myself.

Now, I figured that by scanning the negatives in a fairly high
resolution, I'd get the best results. I have a Canon semi-pro scanner
that works fine (usually). Now I scan these negatives, and on my
computer-monitor they look exellent - much better that the ones I've
downloaded. I figured I'd burn them on a CD and show them using a
DVD-player and a Big-screen projector. This seemingly works fine,
except that the images that I scanned myself look horrible - whereas
the infirior downloaded images look fine!

Is there any reasonable explanation on why this is - and what I can do
to fix it.

And on a related matter - I thought about using the computer as a
player, and just connecting it to the projector, that has a cable like
the computer-monitor - but I get a NO SIGNAL. There's a switch on the
projector that says "tv"/"Computer" and I have it on "Computer". Do I
need a driver og is there some special something I have to activate on
my computer?

Any help would be welcome.

Regards,
Nikolaj Winther