See the "VC80 errors" section in turingtest2's guide at:
Troubleshooting issues with iTunes for Windows updates
where it says:
The message "An error occurred during installation of assembly 'Microsoft.VC80.CRT, type="win32", version="8.0.50727.6195", publicKeyToken="1fc8b3b9a1e18e3b", processorArchitrcture=..." may occur because a particular library file is not included in the distribution package supplied with iTunes or won't install for some unspecified reason. Installing the Microsoft Visual C++ 2005 Service Pack 1 Redistributable Package MFC Security Update before attempting to reinstall iTunes may help.
There are three files to choose from so you need to select the one that matches the processor architecture of your system, which should be noted in the error message.
- vcredist_x86.EXE - 32-bit systems
- vcredist_IA64.EXE - Itanium processors
- vcredist_x64.EXE - other 64-bit systems