During the .NET Framework v4 update installation I received the error code 643. Neither restart nor re-installation did help.
Short search in the internet brought me the following page:
Method 1 did not work for me, however, method 2 pushed me to uninstall some stuff. I uninstalled .Net v1.1 and .NET v4.0. Then I installed v4.0 and the update was installed successfully. It did not even ask for restart. Then I just added v1.1 and all its updates.