Can't run WinXP built-in Defragmenter

I cannot run Windows built-in defragmenter. When I try to acces it, I get this message:

Microsoft Management Console
MMC cannot open the file C:\WINDOWS\system32\dfrg.msc.
This may be because the file does not exist, is not an MMC console, or was created by a later version of MMC. This may also be because you do not have sufficient acces rights to the file.

I get the same message when:
-Start>My computer>right click>Manage
-Start>run>service.msc

Could anybody help me?

WinXP Pro, SP2

P.S.: 30-40 minutes before this happens, for the first time since I use Revo Uninstaller, I run Junk Files Cleaner.

 

Registry editors/junk file cleaners..........
Welcome to majorgeeks.
If there is a backup, as often there is in these programs, choose to use it.
If not, you need your XP disc.
Click start, run.
TYPE IN SFC /SCANNOW, (NOTE THE SPACE AFTER SFC) You may be prompted to click retry several times, but sit there and see it through, and it will replace any protected Windows files.
Come back ifyou need more help - ther are plenty here at majorgeeks to help, if I am not on-line

 

Tried some solutions - including yours - but didn't work. So, I've restored entire partition from a backup image which I created after last Windows reinstall, a month ago.

Thanks for your promptness and for the useful information you gave me!