Try putting the i386 file from your XP CD onto your C:\ drive and altering the registry to point to it there, you can then run sfc/scannow without inserting the CD.
Step 1
You will need to get your XP CD and locate the folder called: I386
This is a major folder and should be one of the first you see, now copy this onto your hard drive into the system root. For most of you that is going to be C: so you should end up with a folder that looks like: C:I386
Step 2
Your computer needs to know that you now have the necessary files on your PC. This is done by editing the registry. Here's how: (type regedit in the Run box on the start menu)
By navigating to:HKEY_LOCAL_MACHINESOFTWAREMicrosoftWind owsCurrentVersionSetup
You will see various entries here on the right hand side.
The one we want is called: SourcePath
It probably has an entry pointing to your CD-ROM drive,(usually D:) and that is why it is asking for the XP CD. All we need to do is change it to: C: Now, double click the SourcePatch setting and a new box will pop up allowing you to make the change to the drive letter of your root drive, probably C:
Now restart your computer and try sfc /scannow again!