Why do I have 45 Gbs of un-defraggable data on my Windows Vista PC?

Using Defraggler, every time I scan before defragging, it shows like 45 Gbs of fragmented data. Then I attempt to defrag it and end up with basically the same number. Any help as to why this is so? I believe there was an update for Vista that solved this problem, but eradicated the ability to downgrade to Xp afterwards, but I may be wrong.