When you click the iTunes icon, hold down shift. This will let you chose what library you use. Direct iTunes to the library in your other OS, and voila.
However, this will create a new iTunes library file in your Win7 folders, so the libraries will not be synched if you add new music.
There was an iLounge article about it for more info, but I can't track it down. Take a look around iLounge if you really want to find it.