Windows update error code 643

Resolution:

Start >> Start Search >> Services.msc
Windows Modular Installer (Started and Automatic)
Windows Installer service (Restart the Service)
Download, Install and Run Windows Installer Cleanup utility from http://download.microsoft.com/download/e/9/d/e9d80355-7ab4-45b8-80e8-983a48d5e1bd/msicuu2.exe
Remove the Microsoft .net framework 1.1
Restart the computer
Download and install the .NET Framework 1.1 from http://www.microsoft.com/downloads/details.aspx?familyid=262D25E3-F589-4842-8157-034D1E7CF3A3&displaylang=en
Restart the computer and then download and install .NET 1.1 SP1 from http://www.microsoft.com/downloads/details.aspx?familyid=A8F5654F-088E-40B2-BBDB-A83353618B38&displaylang=en
Restart the computer
Try performing windows updates again