How to design javascript popup window to show different alert message?

I need to design a popup window to show different different alert message in same popup window.If I use below code

this.Page.ClientScript.RegisterStartupScript(this.GetType(),"ShowPopup","alert('Username or Password is not valid.');",true);

then it opens the popup window but controls get disappear from page. When I click "ok" button on popup window then controls appear, It should not be happen. I want same window with colorful design and when window opens then control should not disapper. Please help me in this.

Thanks

Vishwa

Thanks,

Vishwa