You could display a Portlet in a new window in either of the two ways :-
1) Place the portlet to be displayed in a page and assign a friendly URL to the newly created page. Give this friendly URL in the Target of the link which would open this page in new window.
2) A Portlet could be opened in a new window in SOLO state.When the portlet is opened in this state,it need not be placed in any page.However the portal theme elements would be hidden when its opened in SOLO state. checkout the below link which explains how portlet could be opened in solo state.