I encountered a difficulty to install additional languages on Vista machine.
Here is how to reproduce the problem.
From taskbar Icon, choose 'Program setting...' and 'Language'.
Then 'avast! settings...' dialogbox appears.
But the following link doesn't seem to work.
'To install additional languages, click here.' in the dialogbox.
Fortunately I've found a workaround. If you open the application window, it works fine.
I guess it's something related to User Account Control of Vista.