Good replacement for ShowModalDialogue

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Hi all,

Currently in my website, I am displaying a ShowModalDialogue to display a warning. Recently I have been asked to modify the behavior of this as belows :

Earlier behavior :

I used to just show a warning message. The user can click OK or Close button of the dialogue box to proceed.

New behavior :

I have been asked to insert a text box in the same ShowModalDialogue. Now the user has to insert his initials before clicking OK button. Also now he can not close the dialogue by clicking Close button also. If he tries to close the dialogue by clicking either on Close button or OK button while text box is empty, same pop up has to open with warning. But the trouble is after closing the warning message dialogue is shifted randomly on screen even though its properties have been modified such that it will not move.

Now I have been asked to replace this ShowModalDialogue by something better.
I have two suggestions : 1. CSS div Popup 2. Ajax-control ModalPopup.
Which one of them is better?
Is there anything else which can be used instead that I can use?

My major concerns are good look and easy to handle.

Thanks in advance.