How and where did you download the avast program from ?

Did you use a download manager program ?

Did you try to download and install in one step, rather than download and save to your hard disk and then run the saved file ?

When you installed it were you using an account with administrator priviliges (not a limited account) ?
Did you install avast to the default location (C:\Program Files\Alwil Software\Avast4) or did you choose a different location ?