Actually, I'm not sure Avast starts properly.
The W7 starts in normal mode. Then offers the login for my two users (both with administrative capabilities). Then I click on one of them and the system request the password. Just after entering the password and pressing enter, the system shows the "initializing" screen. A few seconds later, the screen goes to black. I can move the mouse and the keyboard answers to "caps", "numlock" and more.
If I try with ctrl-alt-supr, escape or whatever you could consider to interact with the system... simply does nothing: the black screen keeps on.
I would like to remember I have the avast! Endpoint Protection Suite, managed by the server machine as a SOA. And, perhaps is useful for you, the server detects my PC as terminal. But once logged in (and in black screen), the servers "deactivates" my PC as terminal. So, my conclussion is obvious: once identified in "normal mode" something wrong happens.