I have found that if the output sample rate for the device in the Sound Control Panel is set the same as the ASIO output sample rate (e.g. both are 44.1kHz) then system audio can get mixed into the ASIO playback. I don't know if it happens with all hardware, or just some, but it also happens with my DAC. (Benchmark DAC2 HGC)
WASAPI should be exclusive if you have that option enabled in Media Center (it is by default) though using an exclusive output may not disable the system volume control depending on the driver your hardware uses. Media Center has an option to set it to 100% when playback starts though.
One solution for ASIO is to set the system output's sample rate to the least common sample rate in your library (e.g. 88.2kHz) so that it does not match up to your music.
Another option, which I use, is to send the system audio to a separate device.
I have my system audio going out to my display via HDMI (it has built in speakers) and then back from the display into my DAC via an optical connection.
This way I can still hear system audio through the DAC, but it's on a separate input from USB. (ASIO)
This has the benefit of preventing conflicts when an application has exclusive control over the DAC. I found that I was unable to launch a number of games when the DAC was the default audio device and another application had exclusive control, for example.
It also has the benefit of being synced to the display. With video playback in Media Center you can set a delay to sync up the audio and video, but that's no use for system audio. When it's passed through the display like this, it gets corrected automatically.
It also gives me the option of turning up the volume on the display's speakers to hear system audio, rather than having to change the output device.