CS3 does have some fancy video editing extras, however if you don't know how to rotoscope/mask to begin with, I wouldn't suggest you try to use them. That said, rotoscoping/masking with CS3 on down the line is done frame by frame, ideally with the
pen tool. Be forewarned this can take a long, long time, 5 minutes per frame MINIMUM (and this is being incredibly optimistic). If you've got 24 frames per second and have to mask 10 seconds of footage...you do the math.
There are a number of explanations throughout the forums for methods of masking. In fact, there is a big one right
here - just ignore the AE part and drill down a few posts.