User Account Control (UAC) is a great feature of Windows Vista. With UAC, you will finally be able to run your machine as a standard user as opposed to a computer administrator. If you are running Windows XP, you are most likely running as a computer administrator.
From a security standpoint, this can open the floodgates to spyware, malware, viruses, etc. By running your computer as an administrator, a compromise can easily write to your system files. As a standard user, access would be denied.
There have been attemps to run as a standard user or Least-Privileged User Account (LUA) in Windows XP but it is difficult and cumbersome. I have read several blogs that show you how to do it with special applications but Windows XP was not designed to run this way.
On the other hand, Windows Vista was designed to run this way out of the box. In Windows Vista, when you run as a Computer Administrator, any administrative task will dim out the entire operating system and provide you with a dialog box to elevate your privileges to perform the specific task at hand.
When you run as a Standard User, an administrator password is required to perform the appropriate task. This is a true secure desktop implementation and is a great feature of Windows Vista.
What are your opinons of UAC? Do you like it or hate it? Let me know.
Update: If you are interested in disabling UAC, see the following post, "Customizing UAC in Windows Vista."