If Avant suddenly starts behaving weirdly, your conf.ini (called "abrowser.ini" in versions before 10) file may have become corrupted. To fix this, close Avant completely - including the system tray icon if in use - and open the Avant settings folder using one of the following paths.
C:\Documents and Settings\your username\Application Data\Avant Browser
C:\Windows\Application Data\Avant Browser
Note: 'Application Data' is a hidden folder. If you can't see it, go to the Tools menu in Explorer, select Folder Options and click on the View tab, then select 'Show hidden files and folders'.
Here delete the file called conf.ini (If you only see one called conf it's the same, it just means Windows is set to hide file tags.) On restarting, Avant will create a clean copy and should run like new.
conf.ini stores the preferences you select under Tools > Avant Browser Options. It may be damaged by a system crash while Avant is running. Because conf.ini is meant to preserve your settings over upgrades, reinstalling Avant does not replace it.
If deleting conf.ini doesn't fix your problem you could try removing the entire Avant Browser folder. Beware though that this will delete your feeds, groups etc. unless you copy them to somewhere safe first.