UPDATE
YEEEK! The WMF viewer is what is used to generate thumbnails of pictures on websites and is used by the preview pane of outlook. This means you can get whacked with some spyware just byt visting a website or viewing an email through the preview page in outlook.
There is an unoffical patch to fix the problem but I personally wouldn't recommend installing an unofficial patch.
If you have an updated virus scanner on your pc such as Norton or McAffee it should detect a corrupt WMF being displayed and disallow it before it infects your pc.
If you are REALLY worried you can delete the dll that runs the WMF viewer (shimgvw.dll) though of course this will cause some images to not display and may mess up the Outlook preview pane. The only sure fix is a patch from Microsoft, which, they say, is in the works.
ORIGNAL POST
A new security hole has been found in the WMF (Windows Metafile) file format. This is the file format used by windows picture and fax veiwer. Currently, there is not a pacth for it. There are exploits and viruses cirruclating on the internet that will use this exploit to install keystroke loggers, spyware, and adware on your pc. The best way to defend against this exploit is to update your virus scanner and don't open any .WMF files you might recieve in an email. The only way you can become infected is by opening a .WMF file.
Here is the full story.