Swift Disk Defrag is a powerful program that does exactly that, on Windows systems.
If your system is slowing down it could be because of file fragmentation. This process implies that when running applications or loading files and folders, the system has to do longer and slower searches, making it slow. To avoid this you have diverse defrag tools included in Swift Disk Defrag which has a fast and reliable engine to do this task as quickly as possible.
Swift Disk Defrag is very easy to use, with an information window where the directories, clusters and files that have been analyzed are shown, and also those still remaining. You can also program the defrag process to be done automatically when you want.