Running SFC without a CD ROM

If you run sfc /scannow and get prompted to insert a CD,
there are a couple of changes you might need to make.

Slipstream your copy of WindowsXP with the latest service pack you have applied.
For detailed unstructions on how to do this see:
SlipStreaming a Service Pack into Windows XP

Once these files are on your hard drive, you can simply make a few registry changes to point to those locations.

Start Regedit

Go to the follow locations and change the path to wherever you copied the source files.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\ServicePackSourcePath
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\SourcePath
  the last one might not be necessary
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SourcePath