Re: Disable Avast startup
First make sure that you are logging in as administrator, than go in to control panel > administrative tools > services and disable it from there. I have tried disabling it in same manner and its working, in case if that doesn't work for you than let me know.
Re: Disable Avast startup
I will suggest you to better uninstall Avast and install BitDefender10 Free Edition or Kaspersky's AVPTool. You dont have to worry for updates as its scanning engine get updated several times a day, just have to download the updated scanner while doing scan. If you dont want to use BitDefender10 or Kaspersky's AVPTool than you can check out Malwarebytes - Anti-Malware as well.
Re: Disable Avast startup
Thank you, that did the trick for me.
After disabling the Self Defence mode through the menu > Troubleshooting I was allowed to remove the Avast from the startup using msconfig.