Agree with IronWoods about Norton - I happen to like PerfectDisk. But there are several things that can cause constant fragmenting, especially if you are running NTFS.
First, if you have your PageFile on "C" drive, and have not fixed its size, it will constantly be changing and that causes other files to move around and get fragmented. The best place for the PageFile is on a separate partition dedicated to it and nothing esle. It works and is harmless that way.
Second, NTFS is managed by the MFT system, Microsoft's MetaFileTable. That reserves a big chunk of real estate right in the middle of the drive that cannot be used by applications, or defragged, or optimized. It's size is a function of how many files you have on that partition and how much space is available. In effect, it gut-shoots your drive and causes fragmentation.
You can defrag (I always do a full optimization) now and as soon as you are finished, you will have new fragmentation - it is constantly happening with XP and NTFS. Once your files exceed the space "above" the reserved area, and have to go below it, files get fragmented. You can actually see it with PerfectDisk and DiskKeeper. Both of them are much faster and better than Norton - and the built in defragger is a lite version of DiskKeeper.
O&O I know nothing about.