Vista's biggest security enhancement is "User Account Protection" (or "User Account Control" ) which pops up and asks for user authentication before software can make any administrative changes to the system. But the TweakVista utility can turn off UAP in one click. Microsoft says this is UAP working as intended, because when a user runs TweakVista they are asked to authenticate.
However, James Bannan at APC Magazine asked Microsoft what's to stop a downloaded "freeware game" requiring user authentication upon installation and then disabling UAP altogether? You'll be surprised by Microsoft's response.
News source: APC Magazine