Pretty much all Windows users run with administrator priviledges. Stop doing that! This is the vector by which pretty much all viruses infect your machine (via email, web browsing, etc...). Because you're running as admin, when they execute they have the run of the machine.
All you have to do is take yourself out of the Administrators or Power Users group and become a Limited User (you'll also hear this called LUA or Limited User Access).
As a limited user, you can do everything you can normally do, except stuff that you need to be admin to do (like install programs, etc...).
In the UNIX world, people wouldn't dream of running as root (the UNIX equivalent of Administrator).
If you need to do an admin type thing, log out, then log in as administrator, or temporarily increase your priviledges to that of an admin via something like RunAs (right-click).
Aaron Margosis covers a lot of this from a technical standpoint in his blog.
Of course, there are issues that are still being resolved. Windows traditionally hasn't promoted the LUA thing, the soon-to-be-released SP2 solves a bunch of these issues. Also, a lot of software (unfortunately, including most games) require you to be admin to run (copy protection, dum programming). These are being addressed via developer education.
So, just do it. You'll thank me. And just think, as well as stopping the virus guys from nuking your machine, you'll stop little Johnny from looking through your personal stuff and finding all the porn (assuming you set the permissions on your stuff correctly ;-)