I saw this when trying to install on my windows 7 system.
Even though I downloaded the itunes installer, then right clicked it to run the install as administrator, it still seems that the services are installed with the incorrect permissions.
After the install failed, the apple mobile device still remains installed.
Go to the control Panel
Uninstall the apple mobile device.
Several other Apple products were also present in the installed programs list. Bonjour, apple updater, etc. You can sort by publisher to find the Apple products. This was a new pc, so they were added by the failed itunes install. I uninstalled them as well to be sure all was cleaned up.
I then disabled UAC. good instructions here: http://www.howtogeek.com/howto/windows-vista/disable-user-account-control-uac-th e-easy-way-on-windows-vista/
I then restarted the system.
itunes install worked fine. I ran the installer from the browser, no need to save the file.
I have some more programs to install, so will leave UAC off for a while. :-)