Sorry, but doggeral gives the solution at that links:
Well if its not an obvious option in the program here is what I would do. Generally programs that run prior to the Logon Screen booting in XP use a specific key (good ol' session manager).
Browse to the following location in regedit
HKLM\SYSTEM\CURRENTCONTROLSET\CONTROL\Session Manager
on the right there will be a key labeled "BootExecute"
Doubleclick that key and you should see at least the default value of "autocheck autochk *" which is scandisk. Somewhere below that should be the path to avast (Alwil Software) with options (sorry I can't tell you what they would be or what the line will look like, bit i don't know however it should be obvious that it is avast) but anyway delete that entire line and you won't see it again.
This assumes you are on Windows XP.