If you're having trouble switching from playing Windows audio and playing audio from your DAW, I would first suspect a sample rate mismatch. In Windows, under Sound->Playback, find your audio interface, look at the Advanced tab on its properties. See if you can select a playback sample rate that matches what you have configured in Reaper and/or Focusrite's ASIO control panel.
Even as rock-solid as my RME is, it freaked out once when moving from Reaper to Windows Media Player, and it was a sample rate mismatch that caused it. Their suggestion was to not make the interface the default playback device in windows and instead use the internal sound card as the playback device (easy since I have a small mixer) to avoid such mismatches. I didn't like the dual sound card setup, so I went back to using the RME as the default device and my ASIO device in Reaper. But I record/playback at 48 kHz in Reaper now, and have Windows set to use the same sample rate/bit depth and I haven't had any issues since.