Symptom:

Customers installed Win8 and got the following error:

“We are not sure what happened, but couldn't finish installing windows 8 Consumer Preview. We've set your PC back to the way it was right before you started.”

 

Resolution:

Running setup from the root of the media within an existing Windows installation assumes that you want to perform a clean installation with data migration.

Running setup from the sources folder gives more of a Windows 7-like experience where it asks you what you want to do.