This may be cause the file doesn't exist, is not MMC console, or it was created by a later version of MMC. This may also be because you do not have sufficient access to the file.
The disk defragmenter is trying to run and can't because it is missing on startup. One way to fix it is to run system restore. another way is to run a repair install of XP using the original operating system disk, without reformatting the hard drive. Try system restore first, then get back here and tell us how it went. There are also other ways to replace the missing file, just more complicated.
Open my computer, control panel, scheduled tasks, is the disk defrag scheduled to run? you can delete it from the task schedule. That's one way to stop the message, but it won't replace the missing file.