Photoshop was mentioned above but the full version may be overkill unless you are a serious/pro photographer. It is expensive but is the industry standard in digital photo processing. Adobe put out a smaller version called Photoshop Elements which is a LOT cheaper and very capable. It still has a fair learning curve to it though. Elements is bundled with some cameras and scanners so there may be people with unused copies around who may have them on E-Bay. (Not sure about the legality of that so I may be out of line here).
Paint Shop Pro is in the same vein, perhaps with a bit more capability in some areas and is similarly priced. Some feel it is a bit more user friendly. They both have trial downloads good for 30 days so you could have a look at them. There are also third party on line tutorials if you do a search.