A drive-by installer installs software without _express_ consent of the user. A default of accept or a checked "install" checkbox for a bundled product that is not labelled directly as the software being installed (ie you clicked a download link and it said SoftwareX) then it's a drive-by.
Adobe are doing this now. Last straw, if Adobe Readers vast girth wasn't enough well this certainly is.
Either the download must say "StuffYouWant.exe with CrapwareBloat.exe" or the installation of the later must only be done by non-default action. If these conditions aren't met then it's a drive-by installer and those consciously profiting from it need to check their morals.