to clean, or free up disk space, do some basic 'house cleaning'.   Go to My
Computer>right click local C:>click properties.
Now, there are a couple of tools that will help free up space.
First, click on the 'disk cleanup'  This will take a few minutes to
populate.  When it's done, you will see a box containing a list of all the
files ( temp. type files) that are safe to delete.  Once the  list has been
populated, click on the 'More Options tab and select the option to remove
'all but the most recent restore point'. located in the third box down
called 'System Restore'
Click 'clean up, and hit 'yes' when asked if you want to remove past restore
points.  Then  click 'okay'.  You will then be asked if you want to perform
these actions..or however it's worded.  Click yes. or okay.  This will clean
out your temp files.  Doing these 2 things should reclaim enough disk space
to allow defrag to run.
Before you run the defrag though, run the error checking tool from the
'Tools Tab'  When you hit okay, you will be presented with 2 additional
boxes to check...check those, and then you will be told you need to restart
in order for those to run..go ahead and restart your computer.  The scan
may take about 15 minutes.  If everything goes well, then run the defrag.