What do you have in the Archive folder as it seems that the folders under it are associated with it. Perhaps after you have a look you may be able to delete whatever is preventing its deletion.
It may also contain an existing email account. If it does you may be able to put it back into you email account list without all the associated folders. If it is an email account that you no longer use or need, you can delete it via File, Account Information, Account Settings when that opens you can remove an account.