Vista's User Account Control (UAC) is one of the most criticized new features in Vista. Select a task or utility marked with the Windows security shield, and UAC asks you to confirm the operation (if you are using an administrator account) or asks you to select an administrator and enter the password (if you are using a standard account).
I'll admit, UAC gets old (especially when you're digging through every nook and cranny of Vista as I've been for my upcoming Maximum PC Guide to Windows Vista Exposed), but UAC has also been very useful in figuring out how programs communicate with the system.
Installing some applications (such as Adobe Photoshop Elements 5) triggers a UAC prompt. Why? When you run a program installer that expects a deeper level of system access than Vista provides by default, UAC pops up to confirm the operation. Should you worry in such cases?
I don't worry about UAC prompts when I'm running a program from a known vendor. But if I see a UAC prompt triggered by a program I've never heard of before, or if UAC pops up and asks me to confirm an operation I didn't start, it gives me the chance to say "No" and stop that program from running.
Disabling User Account Control (you can do it with msconfig, the System Configuration utility in Vista) might seem like a good idea, but if you want to keep an eye on possibly-misbehaving programs, leave it enabled. A little nagging isn't going to hurt you – and it might save your computer from big security problems.