Bossy coworker making work hell?

I have a coworker that is completely equal to me in terms of rank. The only difference is that I was given slightly more responsibility a few weeks ago. With in the past 2 weeks, she was also given more responsibility.

I always heard my coworkers say that she is on a "power trip" and thinks that she is the boss. I didn't really see it myself until recently.

For example: We both were the only 2 working last Friday. It was my job to open and close the work place, since I am keyholder. The whole day, she would say things like, "You can do this and this while I go and do this".

She basically acts like she is running the place when in actuality, we are all equal (except for the actual boss).

What I want is to work together toward a common goal; I know what I am doing and I know what needs to be done, and I don't need her trying to boss me around and making me feel like I am incompetent.

Another thing she does is waste time. We worked together a couple weeks before halloween and on a very busy day, she spent the better part of an hour putting together a decoration for halloween. We had several other WORK things that needed to be done, and here she is taking her sweet time putting together a frigging decoration. So of course, that day we had to stay late to do what actually *needed* to be done.

I really like my job and she is making it very difficult for me to like it there. The problem is that the boss and her are very close and the boss really likes her. I know the boss likes me too but I don't think she has quite the shine to me as she does my bossy coworker.

Anyway, my question is: how do I approach this?

I don't really feel comforable discussing it with my coworker, because she is highly oversensitive and will probably go crying to the boss about it. Also, I am so frustrated and fed up with her that I will probably say something very harsh to her and end up making it worse.

What should I say to my boss?