Well, not knowing how the Soft-mod was done will make it a little harder to undo.
Whenever I soft-mod a box, I rename the files and folders I'm going to replace with a different name. I usually put .original on the end of the name.
I would recommend FTPing into the box to see what the 'C' drive looks like. Hopefully the original files are there with different names. If so, you can rename each of the mod files to .mod and rename the original files back.
For example, if there is a default.xbe and a default.xbe.original, you can rename the default.xbe to default.xbe.mod and rename default.xbe.original back to default.xbe. Do that for all of the original items and your XBox should no longer be soft-modded.
You may want to wait until you get the chip up and running to do this, that way, you can still get into your XBox to try to fix things if there are problems after doing the renames. (Otherwise, you'll have to do a soft-mod exploit to get FTP access back if it FRAGS after the renames.)
If you want to post a listing of the C drive, we may be able to help you figure out what all to rename...
-Todd
