I don't know and to be true, don't believe anything is wrong with your account. What could be though, is that under your account, you may have some applications installed that are conflicting with avast!
I'll repeat, it doesn't have to be, but it's good to check. Also, you may wanna try to compare all installed applications (especially security related applications) under your account and also under all other accounts set on your computer.
1) Have you ever had any other antivirus program installed under your user account before ?
2) If yes, which one ?
3) Did you do everything to properly uninstall that antivirus prior to installing avast! ?
Those questions may sound silly and annoying, but believe me, every single info we can get, it could help you, as well as help us to find solution for your problem...
4) What are security settings uder all those accounts set on your PC ?
5) Who has administartor rights ? I guess, it's you, but you never know...
6) under which account avast! was installed ?
7) Is there any chance you can disable test memory on application start when you are logged in Windows under one of those other accounts, and then log off and log on back again under your account ? Try starting avast! again...