.NET Framework 4.0 updates won't install

I am running Windows XP-SP3 and have had no previous problems with any of Microsoft's .NET framework editions, including .NET Framework 4. Recently, several .NET 4 security updates would not install, while some of them did. I have searched the WEB and Microsoft support for solutions, without success. I have run MS Fix It and tried "repairing" the .NET 4 Framework Client Profile with no luck. In desperation, I uninstalled Client Profile and then downloaded and tried to reinstall it. It would not install.
I learned that many other users are having similar problems. I would
appreciate any help you can provide!