There is so much to learn regarding post processing. Sometimes it is a simple conversion from color to black and white. Other times the purpose of post processing is to enhance the original image by adjusting the light, adding some contrast, maybe even sharpen up an image.
For the very basic adjustments I work in Windows Live, a free download, apparently closed down last year (discovered that when I went to look for the link for you) but still working fine for me.
If I want more than enhancement I go to Photoshop CS3
Shooting around the house on a cold winter day.