This software was recommended to me by a friend who is a professional photographer... and it is nothing short of excellent.
It will do most everything that Adobe Photoshop will do (including conversion of RAW format).... and it's FREE! So... $600 vs $0, it's a no-brainer.
The interface is a little intimidating at first but there are many user tutorials on youtube and the web for it! If you download it, don't be intimidated at first glance, dig into it. I only use a very small fraction of its features and am learning "layers".
Be sure that your OS is compatible. I'm running it on Vista. It downloaded and runs well on this OS.
Anyway, if you're looking for a great FREE editing program this one's a beauty: Gimp Site