"WordPress Already Installed" message on an established WP install

Hi there,

We’ve been having some problems with our WordPress install where if we go to the URL (http://www.frenchweddingstyle.com) it brings up the message:

“Already Installed

You appear to have already installed WordPress. To reinstall please clear your old database tables first.”

We haven’t upgraded any of the plugins, or themes etc.

It also happens when trying to access the wp-admin page, so I can’t even deactivate the plugins easily. I can probably do this in the database, but not for an hour or two until I get back to the office!

Does anyone know of what could have caused this and how we get it back up and running?!?

Cheers
P