Disk Defragment MMC does not open properly

Ran into an interesting one today with the Disk Defragmenter.  Got the following error when opening it up:

MMC could not create the snap in. The snap-in might not have been installed correctly.

Name: Disk Defragmenter

CLSID: {43668E21-2636-11D1-A1CE-0080C88593A5}

Found a couple of interesting post that helped me this was the one that solved the problem:

http://www.andreasroom.com/blog/archive/2004/01/18/defrag.aspx

In the end I did the following:

regsvr32 atl.dll

regsvr32 dfrgsnap.dll 
regsvr32 dfrgui.dll 

If that doesn't fix it for you refer to the link above which has all sorts of other solutions.