Error in Log In

There is an error in logging in feature of my website. When I hover my mouse on “Account” tab on the top menu, a login box is displayed below where users are asked to enter their respective user name and password (Please refer the attached image link to see that).

http://omansupply.com/wp-content/uploads/2014/02/Untitled.jpg

However, after entering the correct username and passwords (be it admin or other roles), they are automatically redirected to the WordPress login page (i.e example.com/wp-admin) where users are once again prompted to enter their usernames and passwords.

How to correct this error so that users will be able to login directly using the earlier mentioned “Account” tab option?

Also even when the users (subscribers and other non-admin ones) and are signed in through WordPress Login, they are redirected to the main website. But they are able to see their WordPress dashboard options.

How do I prevent providing these users access to WordPress Dashboard?

Please help. This is very urgent.
Thanks.