Prevent iTunes from starting when iPhone/iPod/iPad is plugged in on Windows
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.
From Preventing Automatic Syncing :
You can prevent iPhone from syncing automatically when you connect iPhone to a computer other than the one you usually sync with.
Turn off automatic syncing for iPhone:
Connect iPhone to your computer. In iTunes, select iPhone in the sidebar, then click the Summary tab. Deselect “Automatically sync when this iPhone is connected.” You can still sync by clicking the Sync button.
Prevent automatic syncing for all iPhones and iPods:
In iTunes choose iTunes > Preferences (on a Mac) or Edit > Preferences (on a PC), click Devices, and select “Disable automatic syncing for all iPhones and iPods.”
If this checkbox is selected, iPhone won’t sync automatically, even if “Automatically sync when this iPhone is connected” is selected in the Summary pane.
Prevent automatic syncing one time, without changing settings:
Open iTunes, connect iPhone to your computer, then press and hold Command-Option (on a Mac) or Shift-Control (on a PC) until you see iPhone appear in the sidebar.
Sync manually:
In iTunes, select iPhone in the sidebar, then click Sync in the bottom-right corner of the window. Or, if you’ve changed any sync settings, click Apply.