Good question Wumply.From what I've gathered so far,the defragmenter first tries to make space to move files to.A file could be spread all over the disk especially if you are low on disk space.Larger files also contribute to the operating system doing the spread thing.Also FAT partitions are more susceptible to fragmentation than NTFS.My guess is that xp is compacting a part of large file and assembling it with the other parts(also compacted)and making space for the whole thing somewhere.From WIKIPEDIA;The presence of immovable system files (or of files that the defragmenter will not move in order to simplify its task), especially a swap file, can impede defragmentation. ntfsresize can safely move these files in order to resize an NTFS partition.Ntfsresize can be found on many Linux live distros.I'm going to try that when I get a chance.