I'm always using a third-party disk defragmenter for Windows machines. I prefer PerfectDisk. But the machines are still slowing down with time. They are placing huge amounts of garbage into the registry. They are adding more and more DLL files to supervise. Windows is using a lot of linear structures where other OS have hierarchical structures. Win32 still have limited amounts of memory allocated for graphical resources and a very strict scheme for splitting this memory between applications. There are a large number of places in the registry where third-party applications can add in themselves for auto-start.

Several of the Linux file systems seems to handle fragmentation way better. I'm not really sure, but I don't think I have ever defragmented a Linux machine. And I do huge amounts of work on them.