I have some problem to defragment my pc

Many would consider Norton better, because you can select the order in which files are stored on your disk. That would best for the "sophisticated" user. You'd want to know which files or programs you'd want to open most quickly, and put them in the most accessible position.

If you have no opinion on that subject, or lack the knowledge to make the decision, its six of one and half-a-dozen of the other. In such a case, i'd use the Windows defrag, just to keep a cranky operating system happy.
the windows defragmenter does just what is says. it reconstructs files fragments. however, this doesn't have anything to do with WHERE it puts that fragment. norton, on the other hand "optimizes" the disk. norton will reconstruct files and place them in specific order according to the probability that you will access it again. the most accessed files are kept contiguous so they can be read without moving the heads around the disk. so ideally, norton will show a more noticeable improvement.