Have you tried cleaning out all temp files first.
=======================

Or closing all other running programs - Ctrl Alt Del> select one program at a time (EXCEPT Explorer) and click on End Task. Do this for each program listed (EXCEPT Explorer).

It may be necessary to run Scan Disc, Standard then Thorough if necessary before Defrag will run.
========================

Or run Defrag in Safe Mode (keep tapping F5 while restarting the computer to enter Safe Mode). It may be necessary to run Scan Disc. Standard, then Thorough if necessary before Defrag will run.

Hope this is of some help.