Why I cant' defrag my computer?

I tried to stard defrag in many way (start menu, run dialog and so on), and never would start. So I tried trough command line and what I get when I run defrag command is:

Parameter is not correct (0x80070057)

followed by the usual description and information about the command. What problem is this? I've tried to change the decimal separator and to launch this fixit but the error is still there. Any ideas?