This has hit the wild, and although everyone's reported it already, there is a workaround and something to note about Dial-a-fix, so I figure I should let the users of my site know about it.
Basically, it is an exploit that affects WMF (Windows meta file — clipart) files via a bug in shimgvw.dll. This is kind of similar to the JPEG exploit from a while back.
More info: http://www.securityfocus.com/brief/89
To sort-of work around this, you can go to Start > Run > and type regsvr32 /u shimgvw.dll. Dial-a-fix *does* register this DLL via the Shell checkmark on the More… dialog, so if you use that section of Dial-a-fix and want to stay protected from WMF files until Microsoft issues a patch, you must regsvr32 /u shimgvw.dll again after using that section of Dial-a-fix. Note that there will be a loss in functionality in various sections of Windows, as shimgvw.dll does a lot of things for XP, not just thumbnails and Picture and Fax Viewer. Firefox is not directly affected, but if you open or save the WMF file when prompted, you will then be attacked (when shimgvw.dll creates a thumbnail of the WMF, or if you open it from Firefox).
BugTraq update:
If the exploit file is named with another graphics extension (i.e. .gif, .jpg, .png, .tif), the GDI library will still read it correctly as a WMF file and execute the exploit. As a result, all common graphics files can carry the exploit.