If you have trouble to install newer version of iTunes or QuickTime you’ve probably searched for the solution and found that you should use Microsoft Cleanup to uninstall the old version (uninstallation of old version causes the failure). The tool is, however, not available anymore. You have to use Microsoft FixIt, then choose the method to “select the solution by yourself” and after a certain time (the application tries to find problem) click on Uninstall and choose iTunes and/or QuickTime.
FixIt: http://support.microsoft.com/kb/2438651
Alternative method is to open Registry editor and remove all folders containing keys with itunes.msi and quicktime.msi.