As much as anyone hates to admit it, it happens. Someone at your office turns against you because they are overly competitive and envious of your accomplishments. This can be found in a lot of different places of business but is especially found in the higher professional levels. However, even the smaller offices and companies are not immune from those employees that are out to get another employee and will stoop as low as to sabotage your work. In order to avoid this kind of thing and end up out on the streets with the 1,000's of other job seekers, you need to have skills to handle these kind of people.
The first thing you should do if you suspect someone is trying to sabotage your work by taking credit for something you did or they try to do something to mess up the work you did, make sure that this is what is really going on. Sometimes if you are under a lot of stress in your workplace you might not be seeing things in the right perspective. So it's a good idea for you to step back and look at everything that's going on with an objective eye.
If after you do this and it become glaringly clear that they are going out of their way to sabotage your work then you need to make sure that you document it. Make sure to identify what they have done or are doing. Save any emails or other types of documentation that can verify that you are right about what is going on.
After you feel you have enough documentation then you need to find a time that you can meet with your boss about the situation. Remember, this is something that could affect your entire career and you need to be able to speak up about it and bring the problem to his attention. Once it is time for your meeting you need to be objective as you possibly can when you lay out the evidence in front of your boss.
Unless what the person did is grounds for dismissal, you are still going to have to work with this person in the future. So it's always a good thing to do to lessen the stress in your office to allow the coworker to come up with an explanation for the behavior and give them the chance to apologize for their mistake and behavior. The whole purpose of bringing this out in the open is to send a message to your coworker that you are not someone that they can walk all over and that you are the kind of person that can handle these types of situations in a professional, mature and calm manner, while still being firm about the issue.
If it turns out that even after the meeting and giving the coworker ample time to come clean and admit what they did was wrong, does not work out and you think that this will continue, you may have to take it to your human resources department so that they can handle the situation from there.