What can I do if other .NET Framework setup troubleshooting steps do not help?
It is really helpful to get your system back into a known state and then re-installing the .NET Framework and any hotfixes or service packs that apply to it:
Here is also a KB article: When you try to install an update for the .NET Framework 1.0, 1.1, 2.0, 3.0, or 3.5, you may receive Windows Update error code "0x643" or Windows Installer error code http://support.microsoft.com/?id=923100