Microsoft provides a tool to block those unwanted updates...

A blocking tool is available for organizations that would like to temporarily prevent installation of Service Pack updates through Windows Update. This tool can be used with:

  • Windows Server 2003 Service Pack 2 (valid through March, 2008)
  • Windows XP Service Pack 3 (valid for 12 months following general availability)
  • Windows Vista Service Pack 1 (valid for 12 months following general availability)