No access to administrator on new server

Hi All,
I've just installed Joomla 1.7.2 to my new hosting server using Scriptaculous and, although I received an 'install successful' message', every time I try to log in to the back-end as administrator I receive an error message (quoted below). As the host had already installed Joomla 1.5, I had to delete that version first, which I did. I then had to delete the existing database as the account only allows one.

I've searched the forum for this error message, or anything like, but haven't found anything so far.

Can anyone give me an idea of where I should be looking for the problem, please?
Thanks,

Ian

Fatal error: Call to a member function login() on a non-object in /home2/effectua/public_html/administrator/components/com_login/admin.login.php on line 64