Meazure - What are the dimensions of that image? How big is this window?
Bluegrams Screen Ruler - A lightweight and configurable ruler tool for Windows Desktop.
On-Screen Ruler - lightweight, free, semi-transparent ruler
Pixus - Pixus is an AIR pixel ruler application, a handy tool for web and interface designers.
A Ruler for Windows - Free on screen pixel ruler software for windows:The ruler easily measures objects on your screen.
PMeter - PMeter is a very handy pixel meter.