I was called in tonight to fix an iTunes installation problem. When the user tried to uninstall iTunes, they got a popup box asking for a network location to iTune.MSI. When they pressed cancel (because the MSI file didn’t exist) it threw the error: “The installation source for this product is not available”. I checked the machine, and there was no iTunes folder, so it was already uninstalled. It was simply that Windows thought it was still installed. Of course if you tried to install the new version — it popped up the uninstaller again — which made things difficult.
Anyways, the fix for this problem is fairly simple, you need to grab “Windows Installer Cleanup Utility” (Direct Download). Install that and launch it, you will get a list of all currently “Installed” software. Find iTunes on the list and click remove. It will pop a warning, click Ok, then click Exit. Now you can properly install iTunes.