Help! Why do straight lines have a sawtooth edge when I insert an image and print from Powerpoint?

[SnowPatrol161]

If I draw a simple rectangle in Paint and print straight from Paint = smooth sharp lines.
If I save that rectangle as a PNG, open the PNG and print from there = smooth sharp lines.


If I do Insert > Pictures > Select the PNG and it gets added to a new Powerpoint slide... When I print I get a blurry image and looking closely at the lines they have a sawtooth edge. I'm not resizing the image after inserting it. What is going on?

Here's a picture of what's happening. http://imgur.com/05x2Z37

I've tried 'Do not compress images in file' by the way, along with many MANY other things over several hours. What is happening to my work?

[judithvg]

Have you tried filling the rectangle with white instead of leaving it empty?
Or saving as jpg instead of png?
Or copying the rectangle from Paint, using Paste Special in PPT, with option Picture (Enhanced Metafile)?