1) What's the best way to approach my coworker about what I found? Her tax return does not belong on that drive and could be found by others. I would like to alert her to this potential breach of her privacy. She and I have a very good relationship, but the very first page, which was all that I saw, was her W-2, so I now know much more about her than she may feel comfortable with.
2) Now that I know what she makes I am not a happy camper.


I should preface this by saying I'm not the guy who negotiated his salary. At the time the economy was still in the dumps, "you should feel lucky you even have a job" was the zeitgeist, the offer was more money than I was making, and I had grown to hate my previous employer. Almost three years later I've consistently seen my responsibilities expand, I fit in very well with the culture which I know is valued at my small firm, and I had a title added to my name, all for which I've received zero extra dollars compensation. Compare this with my coworker who is the stereotypical bad employee - she barely knows how to use the system (a few months in I was teaching her things), can always be found in the breakroom, and generally is not well respected. And she makes almost 18% more than I do. My role is higher than hers on the org chart. I'm in an "essential employee" role, she is a middling to poor admin.
Rationally I know her salary greatly exceeds mine because she's put in 20+ years and the company is generous with annual raises. But emotionally I'm having a hard time reconciling that versus the obvious difference in our individual value to the firm. This isn't just about a bruise to my ego. I also feel guilty about potentially leaving money on the table. I'm in my early 30s with thoughts of settling down, starting a family, and providing for one or more indigent parents. Compensation is a zero sum game. Every penny I leave on the table today is a penny+ (thank you compound interest!

I guess this part is several questions. Is envy getting the better of me? Should I be more grateful for what I have? Or do I have a legitimate concern about my compensation? If so, how does that get resolved?