Ian, I would double to Jon's analysis, as I have just opened your site and still showing same errors.
You have to get in touch with your hosting provider as what I am assuming is, though they did manage to upgrade from IIS 6 to IIS 7, but trust me, there had been A LOT that has changed between version 6 and 7, and chances are, though your hosting providers did manage to upgrade, without knowing the consequences/changes to be made underlying as again mentioning, loads of things have changed, it seems they manage to mis-configure the drive paths as well. if they have moved the site(s) to the default directory, then they should as well mention in the site's configuration, the updated paths.