This is the main page:
Internet Explorer 8: Home page
If you want to download for a specific version of Windows:
Internet Explorer 8 for Windows XP
Internet Explorer 8 for Windows Vista or Windows Server 2008
Internet Explorer 8 for Windows Vista 64-bit Edition or Windows Server 2008 64-bit Edition
Internet Explorer 8 for Windows Server 2003
Internet Explorer 8 for Windows Server 2003 SP2 64-bit Edition or Windows XP Professional 64-bit Edition
If you want to block IE8 in Windows Updates:
IE8 Blocker Toolkit