It's been in the news the last couple of days, Microsoft has reported a new vulnerability in it's web browser, Internet Explorer.  IE 6/7/8 on Windows XP, Windows Server 2003, Windows Vista, Windows Server 2008, Windows 7, and Windows Server 2008 R2 are all vulnerable.

You can find the details of this vulnerability from the following Microsoft Security Advisory:
http://www.microsoft.com/technet/security/advisory/979352.mspx 

Microsoft annouced they will release an out of cycle update for this Vulnerability, that should be released today.

There is a workaround that you can apply to your system to help protect yourself.  In the above link they mention setting the Internet zone security setting to high will protect users from this attack.  You can find this setting in Internet Options, and click on the Security tab.

This one is pretty serious ... reports link this vulnerability being used in the china-based attack on Google's Gmail service last week.