I agree that it looks like the files are in use and cant be moved, in theory you dont need to defrag a disk until it gets really fragmented, but I would recommend doing it on a weekly basis as it does (slightly) improve the performance of the machine, especially since its a 20Gb drive. if you dont do it for a long time the defrag will take ages to complete, it can also be worth doing a scandisk maybe once a month just to verify the physical condition if the disk.