Not sure if applicable or not, but are you connecting them correctly? While the speakers support both analogue & digital, the Dell's onboard sound & related outputs are most likely analogue only, so connect the speaker's analogue inputs to it. The manual should tell you how to connect it correctly.
Are the speakers powering up? If not see 'No Power from Creative Speakers' click here.
Also see Creative's troubleshooting for your speaker click here. G