Windows 7, Error 66A:

1. Go to control panel.
2. Go to Uninstall programs.
3. Uninstall/Change: Microsoft .Net framework 4 client profile
4. Repair it.
5. Run the Windows Update.

http://answers.microsoft.com/en-us/windows/forum/windows_7-windows_update/windows-7-error-code-66a-cant-install-kb2160841/11b6b2c5-e780-4343-9de5-ffa4619f45c4