wait, when you say 'when I invoke QS a small Spotlight window opens' - do you mean when you hit apple-space you get a small spotlight window? i.e. are you sure you are actually *invoking* QS? the default QS activation is OPTION-space. apple-space activates spotlight. a lot of people (including myself), switch those keyboard commands because they use QS a lot more than spotlight hence people will say hit apple space to activate QS, which i can see now would confuse a lot of novice mac users. Maybe you are using spotlight when you think you are using QS? try hitting option-space now and see if you get a window you've never seen before (if so, welcome to quicksilver).

Speaking of activating QS, one cool trick is you can go to the 'command' preferences in QS prefs and click on 'modifier-only activation' and set it up to 'single' 'command'. that way all you have to do is tap the apple (command) button and it will bring up QS. it might seem like you would be activating it all the time by accident because you use the apple key with everything, but you have to press AND release the apple key without hitting any other keys, so i've found i rarely activate it accidently.