Download the update here: http://www.avast.com/download-update
Why are you using SP2 and not SP3?
Please, upload (attach) the avast log: C:\Program Files\Alwil Software\Avast5\Setup\setup.log
(or the shorter C:\Users\All Users\Alwil Software\Avast5\log\Setup.log).
If the file is too big for the forum, post the last 400-500 lines of it.