I use Picasa 3 to view my pictures, but today I just happened to open a picture in Microsoft Paint. MUCH sharper at ALL picture sizes (Zoom in and out). So I compared a few pictures in Paint to Lightroom 3.3 and ViewNX2. Same story. Is paint applying some sort of automatic sharpening? Paint seems to "snap" the pixels in place regardless of where you zoomed to. It just looks much better! If Paint had a slide show function I would start using it! Any idea what's going on here? I'm on Windows 7.
Whenever you see an image at anything other than 100% something called a filter must be applied to produce the pixels you see on the screen from the pixels in the image.
Different filters have different properties and performance. Some make images look sharper, some less, some work best for enlarging, some for shrinking. There are easily dozens of such filters (with variations) and the time it takes to apply them varies.
A number of applications such as Geeqie let you specify which filter to apply as a compromise between quality and performance. When you resize (AFAIK not for zooming) and image in Photoshop there are also several options which you can try out to see the differences. Modern machines can also use the graphics card (GPU) to perform sophisticated filters extremely fast. Only some software take advantage of this (if the machine can do it) but more and more do.
So to answer your question, MS Paint seems to be using a filter with sharpening properties on your system. Note that things like sharpening and noise-reduction are also types of filters, the only difference is that they output the same number of pixels as they get.
It can be due to the resizing algorithm. My guess is that Paint does not extrapolate, so it pixelize your image. This create a sharper view of the picture. If your are opening JPEG, you don't have a bitmap array of pixel, you must use an algorithm to create those from the JPEG compression.
That's still a guess.