Dan, I'd second Martolod's suggestion of Picasa. I've used it for four years and it really is very simple. And totally free (from Google).
It has the advantage of very simple, and reversible, tweaking by the user (adjusting shadow and brightness, for example, which may be useful with old pictures).
i forgot about the that feature...
it does not alter the source file unless you specifically ask it too, but it will store/display the improved file.and it will read a lot of RAW formats ( it quite happily reads and converts the pentax RAW format that i use)