Cannot reload Win98SE

My client’s child wrecked her Windows OS.
I tried to reload from the original Win98SE CD but got error mesage SU0168 ” Your computer already has an operating system installed….”
I reformatted Drive C (all data is in other partitions) hoping to clean out the corrupted files, but upon reinstalling get the same message.
What can I do?
I cannot use RegEdit to clean out registry because I cannot start Windows!