Hey Maple. You have an Office Cow. Mooo.
We had a secretary (also fat and cranky, I wonder if that's part of the problem) that even her superiors were afraid to make waves with, b/c she was just so unpleasant to deal with. We worked around her and she did whatever she felt like. But eventually we got rid of her.
You're the new person (even though you worked there previously). Don't make waves, but do copy this woman's boss on all your requests. If she snarks in an email, copy her boss on your measured reply (keep it strictly business, don't call her out on her behaviour). I guarantee that you are not the only person this woman is pissing off. These things have a way of sorting themselves out. Cows get slaughtered or put to pasture eventually, its just too unproductive and costly for a firm not too.
Hang in there.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh