Cossack. The defragmenter only shifts data to allow the HDD to read it off faster, it does nothing to actually check the integrity of the data on the disk. When using Norton, best thing to do is to have it as the only application running. Launch the 'One Button System Check' and let it run through and do it's checks. Allow it to fix any and all the problems it finds. If there's a problem with the disk, it'll fix it automatically.
Hope this helps,
Aq