Defragment is a process of consolidating fragmented files on hard disk.
Boot defragment helps Windows to load faster. Though by default, boot defragment is enabled in Windows, it's a good idea to verify it's state as follows:
♦Type "regedit" in Start Menu Search box and hit enter.
♦In the registry editor, go to the following key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Dfrg\BootOpt imizeFunction
♦Check the string "Enable" on the right hand side.
♦Modify it to "Y" to enable boot defragment and "N" to disable.
♦Close the Registry Editor and reboot the system.
Source:
tweakwindows