In Windows XP, if you run a "sfc /scannow" from the command prompt and you get errors such as "Insert Disk", this is a registry issue. If you get a "RPC Unavailable" error, this is because the RPC Service is disabled and is most common in Safe Mode.
To Resolve: Edit
1. Run - regedit
2. Navigate to: HKEY LOCAL MACHINE- Software- Microsoft - Windows - Current Version - Setup
3. Change "sourcepath" and "servicepacksourcepath" to equal "C:\".
4. Run sfc /scannow again.