how do you reinstall them
Where to get them from depends on your computer. If it's a brand computer like HP or Dell, go to their support site and find the download section for your specific model. If it's built from scratch, go to the motherboard manufacturer's site and find the download section for your specific model. In the latter case you should also have a CD containing the drivers, but the support site may have newer ones.
As for the procedure, start by ensuring you have the driver installation package available. You might want to try resetting the drivers first to see if that'd do the trick. Go to Control Panel->Device Manager, find the sound device and remove it. Reboot and let Windows reinstall it. If all goes well, that'd be all it takes. If not, read further.
Go to Control Panel->Programs and Features to find if your sound device drivers can be uninstalled from within there. At least Realtek can be found there. Uninstall, reboot and run the installer you've got. If the uninstall fails, install over the existing ones. If there is no sound driver there, don't worry, simply run the installer.