Did you get installation software when you bought the PC? If not you will have to open the case and determine the make and model of the graphics and sound card. With this information you should be able to search and download the driver software from the internet.
I had exactly this prob with a family PC. In the end we found an installation CD that came for the mobo, ran this & Bingo! everything worked fine. Until then hours of searching for & installing drivers failed miserably. Search through all the CDs you can find!