I had a problem with logging into Joomla 1.7.0. Three times I created a web service from the beginning, and each time after several days of work on it I had a problem with logging into the administration panel.
I want to warn all against the use of "Cookie Settings", because soon after that you log in to Joomla.
If you've been in the Cookie Settings typed something and have trouble logging in, you have to change entry in configuration.php file.
Find the linepublic $ cookie_domain = 'yours.entry';
and change thepublic $ cookie_domain ='';
Now should work properly.