I doubt you are expecting too much, I've heard great things about those speakers...and I'm guessing you're not trying to fill a huge room?
As Alvin said, it is very possible that you have some software settings turned down or otherwise wrong somewhere. I would definitely check those first.
However, I believe it is also very possible that the signal that your laptop is putting out from the headphone jack is simply very weak. I don't know the specifics, but I know that your headphone jack is putting out a certain voltage and a certain wattage of electricity (it's signal), meant to power headphones.
As such, if it is a very weak signal, the factory settings on the built-in amplifier on your new speaker system may not be optimized for recieving such a weak signal.
For this very reason, car audio amplifiers (the only external amps I've had experience with) have a variable gain control. This tells the amp what strength of signal it will be recieving from the head unit. Some headunits send a 2.2volt signal, others send a 4 volt signal. The gain would need to be turned up when using the 2.2volt headunit and down when using the 4 volt headunit (relative to eachother). Well, with computer speakers, as far as I know, you are at the mercy of the factory settings (which I'm sure work perfectly under the intended circumstances).
I hope you see how this relates to your situation, because I've realized I'm sort of rambling now
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So what I suggest that you do, as a last resort, but nonetheless a very promising one, is this: Go to RadioShack, or wherever you prefer, and buy a headphone amplifier. This really isnt much of an amplifier at all, but with such low signal strength it really boosts it a lot. Plug that into your headphone output, and then plug your speakers into it. It should cost about $20. I remember one I was looking at at RadioShack a while back when i first got my laptop was about $20 and had an amp and also provided like 3 additional outputs.
Hope this helps!