If you have Windows XP or Windows Vista it is very easy to set up 2 Windows accounts.
When the computer starts up you can log on as "Jack" or "Jill" (or whatever names you created).
Each Windows user then has their own personal "settings" so you can have your own wallpaper, your own browser favorites and so on.
So each of you can have your own itunes settings and store your own music.
You can create a Windows userid in the Control Panel in the section User Accounts.
If you want to know more just ask.