I first heard about it from some photo forum or newsgroup somewehre (I think it was related to digicams or something). I seem to remember people commenting about it as an easy to use, cheaper utility than Photoshop. Others said get Photoshop Elements.
As I understand it, however, Picture Window Pro allows you to use curves and 48 bit files, whereas Elements doesn't (it's been a while since I've used it, tho).
Picture Window probably has less filter and plug-in support than Photoshop, but then again Photoshop is used by regular graphic artist professionals, publishers and the like. Unless you need all of those extras, a cheaper alternative might be nice.
I'd give the free trial version a go and see if it works for whatever you need a photo editor for.
I have limited experience using Photoshop, Photoshop Elements, and Paint Shop Pro for Windows, and The Gimp for Linux (which is free, by the way, and there's also a version for Windows as well, you might also want to give it a try, www.gimp.org).