Here’s a tool that I’ve been using for some time. It makes measuring screen objects easy!  It’s quick loading (I’ve got it on my launch bar) and works with Windows XP and Vista (I haven’t checked other Windows versions.) There’s a free and paid version. I’ve never needed anything but the free version.  You can move the ruler around the screen and its always on top of the objects you wish to measure.  It can be oriented horizontal or vertical and it’s opacity can be adjusted.  Overall I think it’s pretty nifty.  You can Google it or download it from cNet here.