Paint.net is a free, open source image editor built on top of Microsoft's .NET library (so, you may need to install .NET to use it; this is a 25MB or so download, but it's pre-installed on most recent Windows computers).
Paint.net is very similar in capabilities and user interface to a cross between Photoshop and Paintshop Pro. As such, it is far easier to use (and install) than The GIMP.
It's not a perfect Photoshop replacement, but it's amazingly good, and it's totally free.