How to Prevent the Automatic Installation of Internet Explorer 8

Microsoft offers the Internet Explorer 8 Blocker Toolkit to disable automatic delivery of Internet Explorer 8 as a high-priority update via Automatic Updates and the Windows Update and Microsoft Update sites.

http://www.microsoft.com/downloads/details.aspx?familyid=21687628-5806-4BA6-9E4E-8E224EC6DD8C&displaylang=en

Supported Operating Systems:
Windows Server 2003 Service Pack 2
Windows Server 2003 Service Pack 2 x64 Edition
Windows Server 2008
Windows Vista
Windows Vista 64-bit Editions Service Pack 1
Windows Vista Business 64-bit edition
Windows Vista Enterprise 64-bit edition
Windows Vista Home Basic 64-bit edition
Windows Vista Home Premium 64-bit edition
Windows Vista Service Pack 1
Windows Vista Ultimate 64-bit edition
Windows XP Professional x64 Edition
Windows XP Service Pack 2; Windows XP Service Pack 3