Last week I did a Task Sequence deployment on Windows 7 Enterprise with many (Microsoft) applications. During deployment the Task Sequence was failed because of Windows Update error 80070643. The specific step was during Internet Explorer 9.0 Language Pack installation found on Download Center.
Microsoft has the following to mention about this issue:
Windows Update error 80070643 can happen for several reasons. The most common cause is a problem with the .NET Framework installed on the computer. You might also encounter this error when installing updates for Microsoft Office 2003.
Lucky me the solution was found after a few local tries. The Internet Explorer 9.0 Language Pack can only be installed when the Windows 7 Language Pack is installed already. This is described HERE and needs to be installed locally. So lessons learned: Just install the Windows 7 Language Pack before the Internet Explorer 9.0 Language Pack.