If you think you might need to defragment
Try restarting first. It might help, and it's easy to do.
If your disks are almost full, and you often modify or create large files, there's a chance the disks could be fragmented. In this case, you might benefit from defragmentation, which can be performed with some third-party disk utilities.
Another option is to back up your important files, erase the hard disk, then reinstall Mac OS X and your backed up files.
