Try to create a new user using following SQL script
LOCK TABLES `admin_role` WRITE , `admin_user` WRITE;
SET @SALT = "rp";
SET @PASS = CONCAT(MD5(CONCAT( @SALT , "password") ), CONCAT(":", @SALT ));
SELECT @EXTRA := MAX(extra) FROM admin_user WHERE extra IS NOT NULL;
INSERT INTO `admin_user` (firstname,lastname,email,username,password,created,lognum,reload_acl_flag,is_active,extra,rp_token_created_at)
VALUES ('Firstname','Lastname','email@example.com','myuser',@PASS,NOW(),0,0,1,@EXTRA,NOW());
INSERT INTO `admin_role` (parent_id,tree_level,sort_order,role_type,user_id,role_name)
VALUES (1,2,0,'U',(SELECT user_id FROM admin_user WHERE username = 'myuser'),'Firstname');
UNLOCK TABLES;
Replace myuser with the user name you require and @PASS with the password you want .
Please refer following url for more information
Add Magento Admin User Using MySQL Script
NOTE :
Always apply the latest securiy patches released by Magento to be secure and do not use default admin usrl for the Magento admin section
