It might be worth doing a complete removal of iTunes related programs with cleanup in case one of the other things is causing trouble:
Here is a full method, you have already done some of it.
Download a fresh copy of iTunes and the stand alone version of Quicktime (the one without iTunes)
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 –All Programs>>Windows Install Cleanup
http://support.microsoft.com/kb/290301/
Now use the following method to remove iTunes and its components:
XP
http://support.apple.com/kb/HT1925
Vista
http://support.apple.com/kb/HT1923
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.
When you get to deleting Quicktime files in the system32 folder as advised in the method, you can delete and file or folder called Quicktime.
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.
I don't like giving advice on editing the registry because of the potential for trouble and I am not an expert on it.
There are some apple folders in
Documents & settings\youraccount\application data (hidden folder)
Sometime things don't show up correctly in the cleanup utility. If you see and blank entries or numeric entries that look like Quick time version numbers, those cna be deleted.
If you are still on XP SP2 it might be worth checking the right Windows Installer version - but I haven't seen that as an issue for ages. Sometimes things don't show up properly in the cleanup utility with an old version of the Windows Installer.
Go to Add/Remove programs, and have a look at your Windows Installer entry.
does it say "Windows Installer 3.1 (KB893803)"? or does it say something different?