Ok I'll give you my cleanup instructions for itunes. You need to follow these steps exactly and carefully. If you have any questions about anything then please post before doing it.
1. Uninstall itunes and ipod software. If you can't uninstall use the MSI clean utility.
2. Reboot your PC
3. Re-download the itunes installer and QT and save this installer to your desktop
4. Go to C:\WINDOWS\Downloaded Installations. You should see folders like {06EB3288-C5F4-4C73-8EEE-AC798668FF66}
5. Look in each of these folders for itunes.msi and delete the folder. Be careful to only delete the folders containing itunes.msi
6. Clean out your temp folders, Delete as much as you can in these folders except for the folders Cookies, History, Temporary Internet Files.
a. C:\windows\temp (if one exists)
b. C:\Documents and Settings\{username}\Local Settings\Temp
Sometimes installers will pick up old files or won’t delete their temporary files.
Leave the files if you have doubt about deleting the files.
7. (OPTIONAL) Go to
http://www.download.com/3000-2094-881470.html and download the regcleaner. Unpack it and run it. This is a nice utility that will delete broken registry entries and create a file with them. This way if it deletes anything needed you can add it back. Note I never had to do this with using it for many years. Also it doesn't list XP but you'll be fine. I run XP on both.
8. Reboot your PC.
9. Now try to go and install everything again. I'd do QT first then follow with itunes. Remember to disable virus/firewall/privacy/web accelerators before launching the install.
If you launch the MSI Cleanup utility it will list applications in the window. Select the one you want to remove and click ok. This will remove the msi information from your pc. You still need to do the delete steps in the Program Files folder for ipod, itunes and quicktime cause the cleanup utility won't do this.
Again if you have any questions just post back.