Are you running as a member of the Administrator group right now? If so, I need you to raise your hand, hold it behind your head, and give yourself a dope slap! There's absolutely no reason for any developer to be logged in as a member of the Administrator's group.
I've been developing as nothing more than a member of the User group for over two years. When Vista finally works its way out of Redmond, we'll all be non-admins as default, but there's nothing stopping you from starting immediately to correctly use your computer.
A great case for why running as LUA (Least User Access or Least Privileged User Account depending on who you ask) is important is in a new whitepaper from Microsoft.(Disclaimer, I was a reviewer). The paper covers why it's good to be LUA and what are some of the practical steps you need to take to get your work done.
The simple fact that the nasty virus targeted at Windows relies on you being an Administrator to get their hooks in should be reason alone for you to just stop! Since everyone reading this is a software developer that makes us technical support for friends and family. What I've done is set everyone up as non-admins, with a 15-minute explanation as to why, and all those “come over and get rid of the viruses” or “why is my machine slow” calls have dropped to ZERO. That alone should convince you to take the plunge.
There are numerous resources to show you how to run as a non-admin. Start with nonadmin.editme.com as it's a wiki that consolidates all the various spots on the internet that discuss what you need to do. Much of the content comes from Aaron Margosis, whose blog is a mandatory subscription in your RSS reader. [Wintellog]