There are many post on the INet about the lack of Win-7 audio drivers for the Del GX280 XP computer. Similar model drivers didn't work and Vista drivers didn't work either. Dell does not have Win-7 drivers available for this model.
One post said he was successful after installing Win-7, to use Windows Update to get an audio update listed there.
As an alternative, you could consider an inexpensive add in PCI audio that does have a Win-7 driver, as confirmed on the Win-7 hardware compatability list before purchase.