UltraDefrag is an open source disk defragmenter for Windows NT 4.0 and later operating systems. It makes use of the same defragmenting API used by many other windows defrag utilities, both open and closed source. However, UltraDefrag has many features that makes it unique. Firstly it has a kernel mode driver that does most of the work, secondly is the ability to run the application at boot up time in a manner similar to chkdsk.