Is it every Windows sound, or just some of them? For instance, if you log into your user on the computer, do you hear a crackle in the 'welcome music' that plays, or if you go and test your speakers by going to your playback devices, right clicking your default one, and hitting test.
Either way, it's probably some something corrupted with Windows. Try updating to the latest version of Windows, or doing a repair with a recovery disk if it's annoying you.