If you didn't set a password, just try leaving it blank (no password) and hitting enter.
- Lost or forgotten user password - you can try deleting the C:\ProgramData\Alwil Software\Avast5\aswResp.dat file (Vista or Win7 location). Or C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\aswResp.dat (for winXP).
you would have to disable the self-defence module. Delete as admin user (or use Unlocker.exe).
If that doesn't resolve it then a reinstall is possibly your easiest solution.
Are you sure it wasn't one of those darn cats, very clever