The signal coming from your sound card may be able to drive your headphones, but they won't have enough power to drive most floorstanding speakers, which usually don't come with their own amplifier.
This probably won't work unless you have ultra-sensitive horn-loaded speakers, which can be very expensive:
Sound card --> unpowered speakers
This is what you want:
Sound card --> amplifier* --> unpowered speakers
Sound card --> powered speakers
*amplifier can be a stereo or home theater receiver, an integrated amp, or a straight-up power amplifier. Just make sure to get one with enough wattage for the loudest volume level that you will use. Nothing damages speakers more often than underpowered amps and clipped signals.
BTW, what make and model are your floorstanders and your sound card? Someone might be able to recommend a more specific solution if they knew exactly what you had.
Maybe I misread your question and you really want to know if you can physically place the tower speakers on top of the computer's chassis? That's a whole 'nother story.
is it possible to put floor standing speakers and hook them up to a computer.