More info:
About Fast User Switching in iTunes for Windows
How to force iPodService to start in only one account
Use to avoid the logoff problem
Open the account you want it permanently off of, go to the desktop, right click on My Computer, and hit Manage.
Hit Services & Applications>Services>iPodsrv (or iPodservice, it's an older version).
Double click, set the 'startup type' to Disabled, and it won't open in that account.
Rinse, repeat for any other accounts as needed.