Username and Password login in MS Excel 2010

hello.
I am thinking about using ID+password to login into the excel system.
So let's say I have two text boxes:
txtbox1 = Username
txtbox2 = Password

Button1 = Login

In the worksheet called "Users" I have two columns:

User Pass
John 1212
Kerry 2323
Pit 3434
Tom 4545

Column1 = User
Column2 = Pass

Question is - how to make macro working in such way, that only registered users can log in? And in case if login|password don't match each other it should come up with an error box saying smthing like "Username or password are incorrect"

I am not well trained on VBA, so your advice will be very helpful.

With best regards, Vlad.