If adware is user-installed (manually, not by being shown a 'install this program' prompt and a ream of small print), makes it obvious to the user when it is running (via a startup window, tray icon, etc), and is easily uninstallable (i.e. has a start menu group with a clearly visible uninstall), then I wouldn't consider it malware / vandalism.
Otherwise, it's the technological equivalent of someone stalking you in order to phone you up every 10 minutes to shout an advertising slogan at you.
NB: Tiara - adware can be considered malware in that it usually disrupts both the running of a system (gathering user browsing habits and sending it, for example), and the use of it by a user. This is an intentional disruption - adware intentionally intterrupts your system use to make you look at adverts, for example.
If you consider adware which also acts as spyware, that disruption also applies to potential effects of uploading data (i.e. bandwidth usage when sending your personal information to advertisers).