I like irfanview, very good freeware. I do all my enhancing in irfanview, then if I want to do anything clever after the enhancing, i put it through Fireworks (which isn't free). In irfanview, you can view your histogram to see what needs doing.
The GIMP will be most people's free choice - there is a portable version that requires no installation. Failing that, try Paint.net (requires .netframework 2.0) or Studioline Photo Basic
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