You might need delete the wow directory and reinstall from scratch, the BC cd has written some data to the original wow mpqs, and it's impossible to find and remove em all. When you cancel a BC install, it unwrites all this data, when your computer shuts off, the command to unwrite never gets sent.
And on a somewhat related note, do not EVER shut off, or have your comp shut off during a wow loading screen, it will corrupt your NTFS or FAT system, and render your computer useless. (Newer computers system check and fix the corrupted partitions, older ones... don't.) This happened to both my laptop and my sexy comp, the laptop was 5 years old, and did not repair itself, so I tore it apart and sold the parts rather then spending 200 bucks to get it fixed, my sexy comp repaired itself in 5 seconds

Oh, and I'm supposed to be asleep right now, no one saw me post this post ; )