moving from Wordpress to Multisite Wordpress installation

Hi,
I have a client who's single domain installation needs to be moved to a multisite Sub-domain installation. I've backed up her site and moved it into a sub directory for safety. I did a clean install into the root dir and followed the steps up to getting Network Setup enabled under the tools menu.

When I get to this screen I see "Warning: an existing WordPress network was detected. Please complete the configuration steps. To create a new network, you will need to empty or remove the network database tables."

What steps should I take to do this?