Very very strange,
After the last update a new useraccount avast… is added.
I could delete it by running control userpasswords2 but the next restart of the
laptop it is coming back. 
To remove the account open Avast
Select Settings > Software updater
Tick Administrators only box
Reboot and it has gone
Are you running Avast! 8 R2 Beta2 1485 ?
Is it the new account named “Avast! Software Updater” ?
There is an extensive thread about it i the Beta forum here:
http://forum.avast.com/index.php?topic=121082.0
@essexboy
Did you get rid of the Avast! account by deactivating Administrator only box in SU ? If so you should mention it in the thread above. They are going nuts over it.
LOL I already saw you posted . I was coming here and you were posting there.
No beta, win 7 64bit
Your sollution worked for me.
But the manner to solve it is different but clear to me.