How can I send statistics to admin's mail

Hi All,

Sorry if this isn't the right place for this issue, but didn't know where to post it. I want the system to send an email maybe once a week to the site administrator with all the users that have registered during this period and such kind of information or may be to send email to the administrator when new user is registered. I also don't want users to be add only by the administrator or with his approval. Is there any way to do this because at the moment I can;t think of anything. Any help will be useful. Thanks in advance to everyone :)