I've used both. With work, gimp can do most of what photoshop can do. Paint.NET is more like a PaintShop Pro with it's features. Both are good and bad. Both are great alternatives to forking over a couple hundred bucks.
Yerp...get the GIMP. Great place to start if you want to do real editing but don't want to drop the coin for The Shop. It'll be a little weird when you transition to The Shop as the layout scheme is totally different, but the technique is largely the same.
You can always go to adobe.com and download any of their programs free for 30 days. If you end up liking Lightroom 4 or Elements 11, there are several Holiday coupon codes out there to get each for under $100.
Definitely try Gimp. As a warning, I only recommend downloading the program from its own website. Since gimp is open source, people can modify the program as they see fit. The only way to be certain that you are getting the newest version of gimp is by downloading it from their site. Their site is GIMP - The GNU Image Manipulation Program
I do gimp and UFRaw tutorials and would be happy to help you out more. Feel free to find me on: Gimped! Gimp Turorials