Jim Allchin in his longest blog post to date explains the security trade-off's we had to tackle with Vista:http://windowsvistablog.com/blogs/windowsvista/archive/2007/01/23/security-features-vs-convenience.aspx
At the very end he gives 'good, better, best' security configurations for home users and business users that are extremely wise. They may not make sense to you if you haven't used Vista yet - but they are very good (sort of sad to admit that I run in admin approval mode now without requiriing C-A-D - I may change that today since things have 'settled down' on my notebook now and I hardly ever install stuff or get prompted anymore if ever).
Finally - Vista is awesome for parents as Jim points out. The first account that you get in Vista is an admin account (with admin approval mode enabled so you get prompted for admin tasks but don't have to type in the password). As he points out the next account you should create on a kids PC is the 'kids' account (standard user) . . . I have 3 boys and they share a 'kids' account with no password that is a standard user. I use Vista Ultimate's parental controls to restrict what sites they can visit, what programs they can run and what hours during the day they can logon etc. My kids didn't have a PC when their only choice was XP - but now that I can restrict what they can do see or do on the PC with Vista they've got a PC in their room and they love it. I caught my 3 year old in his room the other day using Encarta kids and he was learning about bees . . . he had figured out that if you press F8 or F9 (can't remember which) it will read to you!