Did you do all the steps in the scond article recommended by Jason?It would help if you explained exaclty what steps you have taken.
Usually this issue is solved by cleaning up the installer files with the Microsoft Installer Cleanup Utility. Did you do That?
In case it helps here is a step by step method for removal of iTunes, installer clean up and reinstall.
== uninstall with cleanup ==
Download a fresh copy of iTunes and the stand alone version of Quicktime
http://www.apple.com/quicktime/download/win.html
http://www.apple.com/itunes/download/
Download and install Microsoft Installer cleanup utility, there are instructions on the page as well as the download. Note that what you download is the installer not the program – you have to run it to install the program
http://support.microsoft.com/kb/290301/
Now use the following method to remove iTunes and its components:
XP
http://docs.info.apple.com/article.html?artnum=93698
Vista
http://docs.info.apple.com/article.html?artnum=305409
If you hit a problem with one of the uninstalls don't worry, carry on with the deleting of files and folders as directed in the method.
Restart your PC.
Run the Microsoft Installer Cleanup Utility. (Start > All Programs > Windows Install Clean Up)
Remove any references you find to the programs you removed - strictly speaking you only need to worry about those programs where the uninstall failed.
restart your PC
Install the stand alone Quicktime and check that it works.
If it does, install iTunes.
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