[umbrant]

You have to be intrinsically motivated to succeed at getting a PhD, which normally means being strongly interested in your area of research. It's a lot of hard work; I wasn't kidding when I said I pulled three weeks of 16 hour days in a row (and that paper wasn't even accepted!).

If you've gotten an undergraduate and master's degree in computer science, you should already have a pretty good feel for the different areas. Choose the one you really want to immerse yourself in for reasons of pure interest, and pursue that one. Once you have the PhD, you can go on and do whatever you want afterwards. You don't have to stay in the same subarea as your thesis; lots of professors (my advisor included) have changed areas based on the times.