The Space Bar is a very convenient way to trigger Quick Look in the Leopard Finder. However, if you're trying to select items whose names begin with a space, the Quick Look window will appear and eat your subsequent keystrokes.

To select the first item that begins with a space without triggering Quick Look, hold down the Option key while you press the Space Bar. You only need to do this once; once you've started to type-select, you can use the space bar unmodified.

Type-selection can also interfere with Quick Look. To use Quick Look after type-selecting, wait a moment before pressing the Space Bar, or just press Command-Y immediately.