defrag

I am running WinXP Pro and trying to defrag my C hardisk, I keep getting the message “defrag stopped because of an error with the file ….”. I have gone to the directory listed and errased the file specified and re-booted. I still get the same message and after looking into the directory specified, the file is no longer there. I am mystified as to what is happening, anyone out there with ideas? Sid Kraft