Claire
Sorry I did not see this before. Do you still need assistance with this? The ECW files should already be georeferenced - if you drag and drop one into ArcMap, does it not display in the proper location?
Am I correct that the ECW files are large mosaics, not single images which have never been rectified?
What version of ArcGIS do you have?
If you still need help let us know -
Cody B