The problem usually occurs on iTunes 10, however, it can also be specified in older versions. When it occurs, your iTunes may not stream music to Airport Express or Apple. Usually, reloading doesn’t help and every time you try to turn it on, it will return the same error “An unknown error occurred (-15000)”.
What happens, in reality, is that the Windows firewall is blocking UDP connections to AirTunes. For some reason, the old basic iTunes firewall doesn’t work with the new version.
But are you ready to fix it on your own? If you are a beginner and feel lost, ask someone who is better versed in software to do it for you. Make sure that they know what the firewall serves for and keep in mind, that “Security” is a too general answer.
And for those, who are ready to tackle the problem on their own, here’s the clue.