just do it in safe mode. many things may cause the defrag to restart. anytime files are added, removed, altered, etc, it will restart itself. windows is SUPPOSED to know, but on older os things like a screen saver popping up would sometimes cause it to not just pause, but crash and restart. sometimes it'll just restart and sometimes it'll say the contents on the HDD have changed, restarting defragmentation. So, run in safe mode. Other things can be done, but do that first. Also, scandisk prior in safe mode.
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.