I know what you mean. I ran into the same problem. I have Windows 7 and I clicked upgrade on iTunes like a fool and then it would not run. I tried uninstalling, cleaning, registry updates etc. Anyway, here's what worked.
You have to delete the iTunes folder in the Music directory (Click on the start button, then select Computer, and Music directory is under Libraries). Don't just delete the iTunes folder because you will lose all the music you saved. Copy the existing iTunes folder from the music directory into another folder called Temporary Hold. It may take 20 minutes or more if you have a lot of songs. After you copy the iTunes folder into this Temporary Hold folder delete the original iTunes folder.
Now, uninstall Quicktime using the Windows uninstall in the control panel.
Start iTunes. Click "Agree". iTunes will come back and ask you for your password and you are in.
Close iTunes.
When iTunes starts it will create a new folder called iTunes in your music directory. Just copy the temporary iTunes folder that you created in the Temporary Hold folder over the iTunes folder that Apple created and your songs are back.
Start iTunes. iTunes will flash a message that it is updating the music directory and then you are done.
I'm surprised that Apple did not fix this problem or at least respond. Usually they are pretty good about helping. I wonder if they are attempting to discourage us Windows users. Good luck.