First off, have you tried unplugging the headphones? Many times, headphones will cause the computer/sound card/drivers to disable external speaker output.
Second, are you sure the speaker wires are all in the proper ports? I.e. your front-audio is where front audio comes from, back-surround where back audio comes from, and your center/subwoofer where that should be?
Third, did you install the sound card drivers? You said headphone drivers, but I'm not sure why you would need those, even with USB headphones. If your motherboard has some sort of audio management software (and you installed it), open that up and look at the various settings. You might be setup to use only headphone output. You might not have the back ports turned on. They might be auto-sensing, but not turned to the proper type of device. You might have the volume on the back ports turned all the way down or disabled in the "mixer".
Are you instead using Digital output to run your speakers? Some audio chipsets will not enable both Analog (headphones), and Digital (SPDIF coax/optical) outputs at the same time.
Also, the model of your mobo would probably help too.