Update:
I have had my updates set to manual
I'm on avast Home, dial-up connection, with winXP Pro and the set to manual appears to be working for me or rather it isn't checking for updates and the last update I have is (080505-0) from before I set things to manual. Since the latest is 080506-0 then it didn't check for nor download that VPS. And nothing in the setup log to indicate that it has even attempted a connection to check for updates. See images 1 & 2 none of which show any activity later than my post yesterday.
On dial-up it should effectively check for updates each time you connect if you have the VPS update settings set to Auto.
So I'm at a loss as to what is happening on your system.