SYMPTOMS
On a computer that is running Microsoft Windows XP, Microsoft Windows 2000 Server, or Windows Server 2003, one or more problems may occur after you install the critical update that is discussed in Microsoft Security Bulletin MS05-051.
Source: Jerry's Security Weblog

These problems include the following:

• The Windows Installer service may not start.
• The Windows Firewall Service may not start. 
• The Network Connections folder is empty.
• The Windows Update Web site may incorrectly recommend that you change the Userdata persistence setting in Microsoft Internet Explorer. 
• Active Server Pages (ASP) pages that are running on Microsoft Internet Information Services (IIS) return an “HTTP 500 – Internal Server Error” error message. 
• The Microsoft COM+ EventSystem service will not start.
• COM+ applications will not start. 
• The computers node in the Microsoft Component Services Microsoft Management Console (MMC) tree will not expand.
• Authenticated users cannot log on, and a blank screen appears after the users apply the October Security Updates.
 

For the Cause and Resolution, see:

http://support.microsoft.com/Default.aspx?id=909444