Hi ,
I've had the same problem with old 5.1 sound cards and found 2 possible solutions . Firstly if onboard sound is enabled or in use it will override that pci soundcard . Onboard sound can be enabled / disabled in the BIOS settings .
Firstly though , switch off the pc and open the PC casing . Make sure to touch bare metal on the casing to ground and static charges and remove the pci soundcard . Replace it in a different slot if you've one avaliable , if not place it back into the same pci slot . Leave all plugs out of the card boot the machine and XP should give the " new hardware installed " message . Windows will then install the generic driver . You should then be able to install the driver from the disc if you wish . Plug the speakers back in using the correct output port ( lime green usually ) . I've never found much difference in sound quality with using generic windows drivers but then I've only ever used soundcards for simple tasks .