The error I received was:
"Windows is unable to install to the selected location. Error: 0x80300001"
The cause apparently is the fact that the install disk is missing from the drive. Ok, my bad: I had to take it out (virtually) to install the Virtio drivers. But is it really that difficult to write an understandable error message? What is wrong with: "The install disk is not in the drive.". Sheesh.