It is probably worth checking to see if there are any remnants of the old Quicktime installer left behind. You can do this with the Microsoft installer cleanup utility.
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. The installer doesn't give any message to confirm the installation.
http://support.microsoft.com/kb/290301/
(To run the program – All Programs>>Windows Install)
Run the utility and look to see if there is an entry for Quicktime, delete it if there is one. otherwise look for any blank entries or numeric entries that could be a QUicktime version number, i.e. 7.x
If you find anything to remove, try installing Quicktime again.