How to increase my chance for being admitted to a PhD if my MSc is in a different topic? I have a MSc degree in Computer Science from a well known university, but the problem is that my degree is, for example in Computer Graphics. Now I would like to study PhD in computer Science, but related to another topic such as Green Computing.  

How I can convince the admission committee or the Professor in charge of the research group that I can be a good student in his group?

One friend told me that one important thing is the motivation letter others about the research proposal (which for me is somewhat not so clear, because for what I know one do a research proposal only when one is inside a group)

Any help will be valuable? I have applied to a lot of PhD positions and the frustration is getting higher with each rejection.
The change of topic should not be a serious problem as long as you have a solid CS background.  What is more likely the problem is your choice of new topic, and whether the schools where you are applying have faculty working in green computing.

For PhD programs, your best bet is to send a targeted and well-informed letter to professors working in the field that interests you.  I cannot stress the well-informed part enough - this is more than just looking at the faculty member's web page.  Read some of his/her papers, think up of suggested research directions or potential improvements of the research.  Demonstrate an ability to understand the professor's research and provide novel and meaningful ideas.