In Windows, there is one account / user name that is named exactly like that "Administrator".
Download the following:
http://www.avast.com/uninstall-utilityReboot into
Windows Safe Mode ("F8" just when Windows is about to "kick in" when booting).
In the removal utility, select the correct folder and correct version / edition of Avast.
Repeat this for other versions / editions of Avast you ever had before (
for example, if you have updated some time from Avast 5 to 6, or if you had Avast Free 5 and then AIS 6).
Reboot again (please do not skip the reboot) into Windows Safe Mode. Try running in this mode the setup of AIS 6.0.1367. This is NOT the normal method for the installation, but the things you are describing under normal mode are too strange. Avoid updating or configuring anything in Avast. Simply reboot after the installation tells you it has finished.
If the installation under Safe Mode won't be possible, then try installing under Windows Normal Mode and reboot.
(The setup has changed a little from the previous stable 6.0.1289 to the current 6.0.1367, so I don't know if installing under Windows Safe Mode will work. The removal utility, for sure it will work, and it is much better to run it under Windows Safe Mode).
I won't to stress out that you should avoid any additional configuration or update of AIS, except if AIS does it by itself. So, no even inserting the license for now.
Please report back.