Hi Juvenal,
> I have a moodle 1.7 test site on localhost. I am constantly being reminded to register Moodle and if I do so, I am told that http://localhost is not valid. [...] I deleted the register.php file in the admin folder and I still keep getting this popup. Same with cron which I don't need to use while I go through the testing process.I also have a moodle 1.7 test site on localhost and I do not find the cron maintenance and register site reminders irritating at all. For one thing they are not displayed in a popup window as you mention but only in the Notifications window, which you do not have to visit in the normal course of using your Moodle local site.
Quite frankly you can just ignore these reminders and use your Moodle local install peacefully. And of course you can not register a Moodle site installed on a localhost, since your localhost does not exist on the World Wide Web.All the best, and do relax, man 
Joseph