Hi, my name is Sergio. Thanks for allowing me to help you. It is important to uninstall iTunes using Apple's instructions or it will not be completely removed.
First, uninstall iTunes and the related components by following these instructions: http://support.apple.com/kb/ht1925
Then, completely restart your PC and perform these steps to clean your system.
Ccleaner - this will remove unnecessary files from your PC and free up a significant amount of space, including temporary internet files and other junk files. It will also clean out the system registry, which is where all of your program pointers reside. Run the file and registry cleaner 2 or 3 times each, or until they do not come up with findings..
http://www.filehippo.com/download_ccleaner (click on the green download button on the top right)
Sorry about the numbering. Try these instructions instead:
Carried out all of the above and re-downloaded iTunes. Whilst installing it stopped again with the following message : "Could not open key : UNKNOWN\Components\DA42BC89BF25BDOAF18C3B9B1A1EE8\057978BEDCC3104FB5D20494DADB50D. Verify you have sufficient access to that key or contact support personnel." The installation process continued and then stopped again with the same message but a different key. I cancelled the installation.
This is the same as my earlier attempts. David
Let's try a generic fix for downloading/installing problems: Go to Start / Accessories / Command Prompt and right click on command prompt and clidk run as administrator. Type sfc /scannow (note the space - it's important) and let it run. It scans and fixes your system files and may resolve the problem.