Locate your iTunesPrefs.xml file. It’s usually located in C:\Documents and Settings\username\Application Data\Apple Computer\iTunes or C:\Documents and Settings\username\Local Settings\Application Data\Apple Computer\iTunes.Hint: If the folder Application Data does not show up, make sure that hidden files are visible in the Windows Explorer
Backup your iTunesPrefs.xml file
Open iTunesPrefs.xml using a capable text-editor (e.g. Notepad++, Ultraedit, but not MS Notepad)
Search for a section called User Preferences and paste the following snipped into the User Preferences Section after the first <dict>:<key>DeviceBackupsDisabled</key>
<data>
dHJ1ZQ==
</data>
After you've done that it should exactly look like the screenshot on the left.
Save the file and restart iTunes. Backups should now be disabled. To enable backups again delete the XML Snippet from iTunesPrefs.xml file.