Does you system have the drivers installed for your sound card?
Is it a self build or was it an OEM PC?
Check your hardware device manager and see if there are yellow exclamation markings against your audio hardware.
To do this:
Open Device Manager by clicking the Start button , clicking Control Panel, clicking System and Maintenance, and then clicking Device Manager.
As we know nothing about your PC we cannot help.
Read this and post back with relevant information that might help us help you.
☺ When everything else fails, read the instructions.