Vista's new User Account Control (UAC) is a new security component in Windows. If you've used Vista at all, you know that it feels like Windows Vista nags you about launching every single program. Here's a simple explanation: Microsoft wants to protect your from evil software, so it makes you think twice when you do something Microsoft doesn't recognize; it does so by asking "are you sure?" when you do something like install software or launch a program that is not registered with Microsoft (e.g. Firefox). For a detailed explanation, see this Microsoft Technet article.
Anyway, these prompts can get annoying, almost like nagging. I didn't like them in the beta, I don't like them now. I ran Vista with the UAC prompts for a week, and I disabled them today with the secpol.msc tool (see this URL to disable UAC it yourself).
That's why I found this new Apple "get a Mac" ad so funny... (click to watch)