It's not photoshop, but an extremely well put together free open source alternative:
http://en.wikipedia.org/wiki/GIMP
http://www.gimp.org/downloads/
I've made the switch on my home machine, as you may suspect it's a deep rabbit hole learning a new photo editor, but it's well laid out.
I should also mention Faststone Image viewer is excellent for light photo work:
http://www.faststone.org/FSViewerDownload.htm