If you can get your head around it and you need some seriously advanced image editing features, you can't beat the GIMP.
Originally written for the Linux platform, it is now available for Windows and is free to download: click here
It is, to all intents and purposes, the Adobe Photoshop of Linux, which you can now enjoy on Windows too thanks to some very clever programming.
Irfanview is another freebie which is small and simple to use: click here
It makes a good companion to the program mentioned above by harristweed.