How can I avoid opening a new browser window when my VisualForce backed button is pressed?

[Vegaln1]

Hello I currently have a button that executes a VF page that calls a extension controller on the Case Object. This button provides the Case history for this Case Id when a user is logged in via Customer Portal. Everything works fine except..... when the button is pressed a new browser window is opened. When I click the tab to return to the case it returns to the newly opened window not the original window. If the user presses this button several times the users is left with several windows open.

Is there some parameter/argument ( in either the VF page or the controller extension)  that I can use that when VP button is pressed it uses the same browser window? 

Regards,

[Vegaln1]

Sorry, this was posted in the wrong forum. Pleae ignore