For photo editing, free Windows tool Paint.net is an excellent alternative to Photoshop.
Paint.net is a Windows-based alternative to the Paint editor that Microsoft shipped with versions of Windows. Don't let that put you off, though, as it's a surprisingly capable and useful tool, available completely free of charge. The software started out life as a Microsoft-sponsored undergraduate project, and has become an open source project maintained by some of the alumni.
The focus is on ease of use, and there's a definite tendancy towards photo editing rather than artistic creation. That said, there are a range of special effects available, allowing you to easily create fake perspective, blend and push pixels around the canvas, tile and repeat selections, and so on.
A good range of selection tools, support for layers, and adjustments such as curves and brightness/contrast mean that Paint.net is a great alternative to Photoshop for photo editing, especially if you can do without some of the more recent additions to Photoshop's toolset.
Paint.NET - Free Software for Digital Photo Editing
