You've already probably heard by now the many efforts and changes which will be coming into place with the release of Windows Vista later this year regarding how "least privilege" (I use quotes as the actual term has changed a few times and I'm not sure that the latest TLA is) will be strongly enforced through all interactions between user and computer.
However, applying least privilege to user accounts in Windows XP isn't always the most pain-free time to be had - users can be frustrated, support calls 24x7. You know the story, I'm sure. However, yesterday the MSSC (Microsoft Solutions for Security & Compliance) team released "Applying the Principle of Least Privilige to User Accounts in Windows XP". Interesting reading.