You need substantial free space on the hard drive or defrag won't run.. Is your free space OK?
Some are fixing the issue by running Error Checking/Chkdsk first.. Have you done so? (Although the article below is for XP, it's basically the same in Vista.)
How To Run 'Error Checking/Chkdsk' On Win2000/XP
Tried running a command line defrag? See the "Manual Defragmentation" section of the link below:
Hope this helps.
The Windows Vista defragmenter suddenly doesn't start any more. I know you cannot follow what it is doing etc., but it really doesn't start at all any more. What can be wrong? I should probably use another tool anyway, but I would just like to know what happened so suddenly???