I just downloaded iTunes 10.5 and tried to install it on my Windows 7 64 bit machine. What I got was an error message that said:
There is a problem with this WIndows Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor.
The fix that allowed 10.5 to install correctly was very simple. Simply go to Control Panel > Uninstall a Program. Right-click on “Apple Software Update” and choose “Repair.” After that, iTunes 10.5 (x64) should install with no problems at all. Apparently the Apple Updater can get messed up and cause the new version not to install. Imagine that.
If only all Windows errors were so easy to fix!