283340 - Chkdsk in Read-Only Mode Does Not Detect Corruption on NTFS Volume
If you start Chkdsk using the method of right-clicking your Drive icon/ Properties/ Tools/ Error Checking, you can select check boxes to automatically check for, and fix any found errors.
Some other process maybe trying to access the drive. The process of writing this file places a lock that prevents Chkdsk.exe from gaining exclusive access to the disk. Because Chkdsk.exe does not have exclusive access to the disk, Chkdsk.exe cannot run. To Isolate what is running at Start up use the Msconfig option. This might help you too.
Windows XP Professional starts logon scripts, startup programs, and services referenced in these registry subkeys and folder locations:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Runonce
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\policies\Explorer\Run
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows\Run
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\RunOnce
%systemdrive%\Documents and Settings\All Users\Start Menu\Programs\Startup
%systemdrive%\Documents and Settings\username\Start Menu\Programs\Startup
Description of the Windows System File Checker Tool (Q310747)
I have seen some anecdotal references to a problem of running SFC and replacing newer files with older versions of files but nothing definite.
If you look in your Event Viewer (Application and System) you should find mention of running SFC and mention of any files replaced. The fact that SFC asked for your XP CD could be that it was just checking and comparing files.