Are you looking for something like this?
map.infoWindow.show(map.position,map.getInfoWindowAnchor(map.position));
That should anchor the info window to a feature position on the map. Or you can anchor it to a mouse click, like this:
map.infoWindow.show(evt.screenPoint,map.getInfoWindowAnchor(evt.screenPoint));
For the infoWindow.show(point, placement) method, the point parameter is an esri.geometry.Point object. It is defined by the x y coordinates of the location. Therefore, it always fix to that location on the earth, not the screen. When the map is moved, the location is moved, and the infowindow is moved too.
You may want to try some other control, e.g. floating window or others, instead of infowindow.
If you are trying to create an InfoWindow that is independent of the map and fixed on your screen, you may want to use a different Dojo Dijit that is outside of the map functionality. You can still populate it with the map derived data, but it will not be anchored to the map. I don't know the exact details of how to implement this, but I have an application where I populated the map attribute data into a DIV on the side of my map and it was not that difficult to accomplish. I imagine it would shouldn't be too difficult to have a fixed popup appear, especially if you can find a Dojo Dijit that already exists for this.
Good Luck!
but if the placement is the archo to the map is it? if i want to click on the icon and the icon move to the side of the map and the infowindow pop up and take up almost half the map view...and the infowindow is fix...how can i do it?
The placement parameter means the placement of the InfoWindow with respect to the graphic. It is relative to the location of your icon. It just means put the inforwindow on the UPPERRIGHT side or other side of the icon. Not the placement on the screen.
http://help.arcgis.com/en/webapi/javascript/arcgis/help/jsapi_start.htm#jsapi/infowindow.htm#show
Like the answer from mflawton, you may need to find another Dojo Diji control. Maybe a Javascript popup box or other better one.
http://www.w3schools.com/JS/js_popup.asp