Just on general principle it's smart not to log in to your computer as the Administrator unless you're purposefully doing something that requires Administrator access, and now it appears like there another good reason to limit your own privileges during normal use.
On Tuesday, December 27, 2005, Microsoft became aware of public reports of malicious attacks on some customers involving a previously unknown security vulnerability in the Windows Meta File (WMF) code area in the Windows platform.
Basically that means that simply viewing an infected image file can corrupt your system, whether you view the image on a webpage, or through email, or whatever.
An attacker who successfully exploited this vulnerability could only gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
Which means that if you're logged in as an Administrator or a user with administration privileges your computer will be much more at-risk than if you're logged in as a normal user.