i use Irfanview, a free download, as my default viewer. it also offers some editing/processing capability such as resizing and adjusting saturation, contrast, sharpness, etc.,though it's nothing like Picasa, Photoshop, or the others whose main purpose is processing. formany pics, it does all the processing i need; others sometimes require a little more work, so i use the "real" photoediting software for those. Irfanview is fast, simple to use, and works very well. italso shows images just as they were saved.
http://www.irfanview.com/