You shouldn't need to download the full VPS update file, unless your system is off-line and you are downloading using another computer to get it and transfer to off-line computer ?
You should be able to get auto updates or do a manual update, right click the avast 'a' icon, select Updating, iAVS Update. These only download the updates you missed since the last update. So are you experiencing problems/errors with these as well ?
As a common user I don't know if you can update in this way, try running it as the administrator, right click on the file, select Run As, from the pop-up window select the Administrator account, enter the password and that should get round any permissions. See example image.