Have you tried disabling "Itunes Helper" (ituneshelper.exe) ?
It's a program launched at startup. To disable it, use msconfig or any program that helps with managing startup programs (Startup Control Panel for instance).
You could also use iTunesBooster (not free though). From their FAQ:
Why would I want to disable / delete iTunesHelper?
iTunesHelper is a service used by iTunes to determine when an iPod is plugged in, so it can automatically launch iTunes. The iTunesHelper service unfortunately eats up a non-trivial amount of your computers available processing power - all of the time, not just when iTunes is running. iTunesBooster disables and deletes iTunesHelper, thus removing the significant performance impact that installing iTunes may have placed upon your computer.