You might receive “Error code 0000066A Windows Update encountered an unknown error” when Windows Update fails to install: Security Update for .NET Framework 4 on Windows XP, Windows Server 2003, Windows Vista, Windows 7, Windows Server 2008, Windows Server 2008 R2 for x64-based Systems (KB2160841).
Use the following steps to fix this issue
1.Go to Program and Features, click it. and Scroll down to Microsoft.Net Framework 4 Client Profile, right click it.
2.It will give you a choice of Uninstall/Change, click it.
3.Then, it will give a choice to Repair it, pick Repair it. It will take about 4-10 minutes to repair (with my PC) your experience may vary.
4.When completed, go to your update center, and update it again.
Remove Microsoft .Net Framework 4.0 with the .NET Framework Cleanup tool reinstall the Microsoft .Net Framework 4.0 and install the Update.
You can download .NET framework cleanup tool from here