Go to the Start > Run dialog box and type in dxdiag and click OK. It should tell you the manufacturer of your PC and by clicking on the "Sound" tab, tell you what kind of sound device you have.
Ordinarily, you go to the PC manufacturer's website (which should be safe), locate and download the relevant drivers and install them. Keep in mind after reinstalling Windows you have to reinstall other drivers besides your sound driver, for example, chipset drivers, graphics drivers, LAN drivers, Modem drivers, etc. Whatever applies. You should also find instructions how to do this on their websites. However, if you still have trouble you can post questions here.
For the sound driver specifically, (if you have onboard sound) you could go to the onboard sound chip manufacturer's website but they usually recommend (at least I think Realtek does) going to the PC or motherboard maker's websites since their drivers take into account any motherboard specific factors.
Edit: AMD-Athlon is only the name of the (CPU) processor manufacturer, not the name or model of the PC.