If your fathers comp is an XP comp then go to the start menu > control panel > system > click on the hardware tab > device manager. Then in the device manager make sure that under the sound option, or sound card, which ever it is, you make sure that the new sound card is enabled and that there are the newest and most up to date drivers. Then you want to disable the onboard sound. Then do a restart and see if that works.
If it is a vista comp then click on the start menu > control panel > system and maintenence > device manager. Then go through the same process of enabling and disabling. Then see what happens.
Cheers!