It can't show more detail if there's no more detail in the image. Once you expand a bitmap past its natural size, you will inevitably see a less clear image. Interpolation won't help you - that's just a fundamental limitation with bitmaps.

Interpolation doesn't show more detail. It is simply the process that happens when you take an image and enlarge it. If you enlarge a bitmap, you're trying to make it fill an area with more pixels than the image has, so you have to add some extra pixels to make the image large enough. The process of inventing these extra pixels to fill the space is called interpolation.

The simplest interpolation style just replicates pixels, and that leads to the blocky 'pixellated' images. (Depending on your definition, you might not even regard this as interpolation, since it doesn't really invent any new pixels, it just duplicates existing ones.)

More sophisticated interpolation schemes try to make a better guess at what would be in those extra pixels if they had been present in the original image. This tends to involve deriving a mathematical function which matches the original image, and then using that function to fill in the blanks. Biiinear and bicubic interpolation are the most common.

As it happens, Windows Picture and Fax Viewer does use interpolation. (At least it does on Windows XP - what are you using?) Not sure whether it's bilinear, bicubic or something else, but it's definitely not doing straight pixel replication.

The fact is though, that regardless of what interpolation scheme you use, if you expand things enough you're going to hit a fundamental limit: interpolation can't recover detail that wasn't in the original image to start with. It can only work from the pixels in the source image. So with any interpolation scheme, if you expand the image enough you're eventually going to see the pixels in some form or other. Interpolation can merely smooth out the edges a bit. It can't magically recover microscopic detail from an image taken with a crummy 200x200 pixel mobile phone camera.