You may wish to think again............. this is a FAKE alert.
Look at the search results here :
Beware of the KB910721 spam email
This morning I received some spam in my inbox that was especially disturbing. It wasn't your usual type of spam that helps increase or ...
http://www.dscoduc.com/.../Beware-of...pam-email.aspx - Cached - Similar
Fake Microsoft Security Alert - KB910721 | SophosLabs blog
17 Jun 2009 ... Yesterday afternoon our spam traps caught a piece of malware disguised as a false security alert from Microsoft: Update for Microsoft ...
Fake Microsoft Security Alert - KB910721 | SophosLabs blog - Cached - Similar
KB910721 - Google Search
KB910721 was an update from 2005.