GIMP, as sugested above, is excellnt but (just like the 'full' versions of Photoshop, which it seeks to emulate) it involves a masive learning curve if you're to get the best from it.
Irfanview deserves a place on everyone's compute, if only because it's a far better picture viewing program than the rubbish that comes with Windows. However it's got loads of photo editing options built into it as well:
http://www.irfanview.com/
Serif is a superb (British) software publishing company, that produces excellent programs that are easy to use. Their full photo-editing program costs over £50 (and is still a bargain!) but they make a 'cut-down' version available completey free of charge; the average user won't actually miss the 'bells and whistles' of the commercial product. I highly recommend it:
http://www.serif.com/free-photo-editing-software/?MC=FSSPHOTOPLUS