Perhaps you already know about Irfanview:
IrfanView - Official Homepage - one of the most popular viewers worldwide
Good for cropping & minor adjustments, reads both JPG & DNG files (and lots of others too if you install the optional plugins), And it's free!
It can also work with many Photoshop plugins. For example, you can do curve adjustments with Irfanview if you install SmartCurve (also free):
SmartCurve
And it seems to work fine with the Noiseware plugin (not free) as well:
Imagenomic - Best Plugins for Adobe Photoshop, Lightroom and Apple Aperture
I do most of my PP on a Mac, but have a very old (windows 2000!) laptop with Irfanview installed, and it works quite well with not much memory.
You might also want to look into Gimp (also free software). The Mac version includes greycstoration noise removal, not sure about the windows version...