I know what the email said, but you have to remember that when you use the on-line registration form, when they email the license they are going to send you instructions in how to enter it manually. Unfortunately it doesn't look like those instructions have caught up with avast 8.
The more important thing is the error message window.
This 'The AAVM subsystem detected an RPC error." has in the past been associated with another antivirus still installed or remnants of one.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
####
You could try a repair of avast (though if the error is related to another AV, it is unlikely to work):
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.