There is a new bagle worm that uses an exploit in IE. It does not need to have the user open an attachment, they just need to read an email with HTML in it. !!!
There is a test to see if you are vulnerable here:
http://secunia.com/MS03-032
More detailed info here:
href=http://secunia.com/advisories/9580/
Bagle Q info here:
http://www.sophos.com/virusinfo/analyses/w32bagleq.html
Go straight to the patch, here:
http://www.microsoft.com/technet/security/bulletin/MS03-040.mspx