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.
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 snippet into the User Preferences Section after the first
Save the file and restart iTunes. Backups should now be disabled.
To enable backups again delete the XML Snippet from iTunesPrefs.xml file.