Thomas Greene of The Register has a fairly comprehensive review of Vista and IE7 user security measures. The verdict is: better but not adequate, and mostly an attempt to shift blame onto the user when things go wrong. From the review: "[Vista is] a slightly more secure version than XP SP2. There are good features, and there are good ideas, but they've been implemented badly. The old problems never go away: too many networking services enabled by default; too many owners running their boxes as admins and downloading every bit of malware they can get their hands on."
The amount of time between slipping on the peel and landing on the pavement is precisely 1 bananosecond.