Doh!
Alright, check your services again, and see if your Avast Management Server service is running and set to automatic.
If it's not running, it can't authenticate you.
If it runs, then stops, then I can almost guarantee it's a problem with the license.dat file. For some reason, I keep having that problem on 2008 machines (I assume it's the same problem on Vista), and I have to rename the license file, then name it back.
Something about the way that the new OS tries to prevent the system from being damaged, but it protects it too much I think.