select by location - Selecting features within Polygon from another layer using QGIS?

I'm using QGIS 1.8 with a PostGIS database and I have three PostGIS-layers in QGIS a lines layer, a points layer and a polygon layer.

What I'm trying to do is to select a polygon in the polygon layer and mark all points within this polygon.

For me the only way to do this is to use the button "Select by Polygon" but in this way, I cannot use an existing polygon for selection.