Well, you can refer to this way:
To repair the .NET Framework you must uninstall multiple versions of the .NET Framework by using the .NET Framework Cleanup Tool, and then reinstall the components.
1. Download the .NET Framework Cleanup Tool:
.NET Framework Cleanup Tool User's Guide
2. Using the .NET Framework Cleanup Tool, uninstall the .NET Framework 1.0, 1.1, 2.0, 3.0, and 3.5.
Note The cleanup tool does not let you remove the .NET Framework 2.0 in Windows Vista or later versions because the .NET Framework is installed as an operating system component.
3. Restart the computer.
4. Download and install the following components:
.Net Framework 1.1:
Microsoft .NET Framework Version 1.1 Redistributable Package
.Net Framework 1.1 SP1:
Microsoft .NET Framework 1.1 Service Pack 1
.NET Framework 3.5 SP1 (this will also install .Net Framework 2.0 SP2 and .Net Framework 3.0 SP2)
Microsoft .NET Framework 3.5 Service Pack 1
5. Restart the computer
6. Visit Windows Update, and then check for and install updates.
The source:http://www.windowsxpregistrycleaner.com/xp-error-0x80070643-update.html Answered May, 21 2011