Top right of the render window - three icons- the left one should be a picture of a camera, which is usually the "render" button.  When rendering in the background it should have a red "X" on it - click this to cancel a render.

My self-build system - Vista 64 on a Kingston 240GB SSD, Asus P5Q Pro MB, Quad 6600 CPU, 8 Gb Geil Black Dragon Ram, CoolerMaster HAF932 full tower chassis, EVGA Geforce GTX 750Ti Superclocked 2 Gb, Coolermaster V8 CPU aircooler, Enermax 600W Modular PSU, 240Gb SSD, 2Tb HDD storage, 28" LCD monitor, and more red LEDs than a grown man really needs.....I built it in 2008 and can't afford a new one, yet.....!

My Software - Poser Pro 2012, Photoshop, Bryce 6 and Borderlands......"Catch a  r--i---d-----e-----!"