hmmmmm. the fact that you're getting that message and that you
can uninstall iTunes from Add/Remove programs is actually a bit exotic. typically, with that one, folks will get a missing iTunes.msi message if they try to uninstall.
we'll work on the hypothesis that you've got skunky installation configuration information for your iTunes, which is causing your Windows Installer to constantly forget that iTunes is installed.
so, let's try the following procedure.
first, head into your Add/Remove programs and uninstall your QuickTime. if it goes, good. if it doesn't, we'll just attend to it when we attend to itunes.
next, download and install the Windows Installer CleanUp utility. use it to clear any existing itunes and/or QuickTime installation configuration information from the PC.
Description of the Windows Installer CleanUp Utility
next, use the the following document as a guide for manual removal of leftover itunes and quicktime program files:
Removing iTunes and QuickTime for Windows
now try another itunes install. does it go through properly? if so, launch itunes, close it again, and then launch it again. does it launch the second time, or do you get the "... only valid ..." message again?