If you have lost the admin password and need to reset it you may have come across this post http://our.umbraco.org/forum/using/ui-questions/4790-ouch-forgot-admin-password-how-to-reset
The best technique to use is this one:
Run this SQL query on the database (or edit the table manually)
UPDATE umbracoUser set userdisabled=0, userLogin='admin', userPassword='default' where id=0
Now search in the web.config the UsersMembershipProvider and change the
Now you should be able to login with admin / default
If that works, revert the change in the web.config and save the web.config. Now change the admin password by using the umbraco-client (users/administrator/reset password)
Check the database to see that the password is hashed again.