Thank JM, although I didn't want to fanny around with other software when the OS is (was) quite capable of taking a quick shot of a window or selected area by holding a couple of keys.
Leopard comes with 'Grab' which is essentially the same thing but you have to open the app first. I know it's no biggie but having to find and open an application when you could have just pressed a couple of keys is rather annoying, imo.