I found this post that might answer your question
http://forum.avast.com/index.php?board=2;action=display;threadid=111
Also try pressing and holding f8 as the computer is loading up. You should be able to get to the safe mode. However, uninstalling Avast has proven to be a challenge for some, I found the easist way is to double click the installer which will ask you if you want to remove Avast once it sees that it is already installed
Hope the thread above helps
James