The biggest problem with UAC is that they tried to clone a process that works quite well in the Unix/Linux world and throw it into the Windows environment willy nilly without taking into account two key factors: The first being that *nix was built from the ground up anticipating that the "super" user would not be "normally" logged in, and the second being that everyone from the newest newb to the most experienced developer knows and anticipates this.
When I try to run a Linux command that requires root access - even when I do so through one of the Desktop interfaces - I get prompted before the tool will even run that I need root access. The developer knows the process requires it, and prompts for it before starting, and even an absolute newb can figure out how to deal with it, and even the most inexeperienced user can pinpoint with near absolute certainty EXACTLY what it is that's asking for permission.
With Vista with UAC enabled, you get cryptic messages that pop up - often times seemingly at random - asking for permission to do something that's so poorly defined that at time even with 25 years of experience in PC technology you're clueless as to exactly what you're being asked to permission and why it popped up, let the 80% of the user base that has less than 20% of the experience. The developers never expected it, the software doesn't anticipate it, and as a result it does more harm than good in just about the most confusing way possible.
Personally, I think MS did this as a bogus "cover our a**" move because the guys at MS aren't exactly morons and only a moron would expect that the average user could deal effectively with it - trying to get off the hook on a technicality that if the user has to permission a trojan to take over their system then it's not "really" MS's fault that the environment is so open to exploitation.
ALL IMO of course.
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