Jason,
That would be a registry entry -
I don't recall if the keypath is different in embedded then in XP Pro,
however if you have regedit installed on your image, open and do search for
"d:\i386" <of course substitute d with the drive letter that pertains to the
CD-ROM>.
I believe it is:
HKLM\Software\Microsoft\Windows\CurrentVersion\Setup
The keyname is "SourcePath"
You would modify the key to c:\windows\i386.
Hope this helps,
--Bill