Suddenly my newly created installer gave this error on the Virtual PC:
Please wait while the installer finishes determining your disk space requirements
I solved this by moving the file I added (and after which this error occurred) to another media file.
Add the second file after the first file:
<Media Id="2" Cabinet="SecondFile.cab" EmbedCab="yes"/>
And refer to this media file in the "File" element:
<File DiskId="2" ...