How to open a popup Window just after logging into the portal

Dear Experts,

I have a requirement in which I have to open a pop up window just after the employee has loginn to the portal,

I tried to use the Confirmation Dialogue, But the problem is , the user can skip the confirmation dialouge by clicking on othr roles or services, I want something like the normal windows popup which can not be skipped.

Regards

Upendra