If you are running XP / Windows 2000 / Windows 2003 - you MUST apply the update on http://www.microsoft.com/technet/security/Bulletin/MS05-051.mspx to stay protected against a critical vulnerability - after you install the Microsoft Security Bulletin MS05-051 - any of the following problems may occur (as per the above mentioned bulletin - Systems that have been modified to non default Access Control List settings on the %windir%\registration directory may experience unexpected behavior)

• 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.

Please take a look at the following KB for a resolution to these problems - http://support.microsoft.com/?kbid=909444