Windows error wont allow defrag

I have been trying to free up some space on My Crive by removing some unused programs etc...and decided to run a defrag after cleaning my registry and doing a complete virus scan with Euseing Free Registry Cleaner and Norton Antivirus respectivly.
At 33% in the analysing part a window appeared with the message that an error occurred in C:WINDOWS\Prefetch\MSNMSGR.exe-0EBDBC56.pf and cant continue the defrag.
Can anybody tell me what this is and how I can fix this problem?
Every serch I did online with the regular search engines came up with no information and a bunch of foreign language sites.
Thanks in advance for any help.