Dave the gnome
You sound pretty young. How old is this co-worker?
Is she finding it difficult to get in with you all?
Does she feel left out?
Is her self esteem very low and is she feeling insecure?
Is she doing the job because she has no other choice?
Is she beeing critisized for her work quality?

If you are prepared to do it, I think you need to confront her away from your workplace and without the other members present.

If it was me, I would be doing the above recommendation, and then starting off by saying how much I think she has a roll to play within the group, but It's difficult to understand why she is slagging everybody off. Is there something that I can do to help you.

Hopefully she will open up and tell you why she is doing it.

In my humble opinion, it is important for her to realise that you find everybody within the team great, and that you are not prepared to take sides, but you are willing to help and advise her.

If it doesn't work then I think you have to make it perfectly clear to her that you cannot take her side. That your team mates are more important. That in future you will work with her, but you are not prepared to listen to her negative attitude anymore. That she needs to buck up and start to work with the team.
If it carries on, you then warn her that you are going to report her to your boss, becuase it is affecting your work.

If she is a nasty peice of stuff, then watch out for the flack, and make sure you cover your backside.

Heart on my sleeve approach to your problem. Hope I have been helpful.
Sorry if I haven't.