I have separate login accounts for my girlfriend (who refuses to own a computer for unknown reasons) and myself on my macs, and she just logs in to sync her iPod & iPhone (check her email, store her bookmarks, and everything else) without messing up any of my stuff.
I assume iTunes behaves the same way on a PC, since all it's libraries are in the "Documents and Settings" folder of the user account directories.
I'd create a separate user account in windows, and log in as different users to keep not only your iTunes stuff separate, but everything else (bookmarks, etc).
On a mac, you can share a music folder (with separate user libraries, playlists, etc) by setting up a shortcut to the iTunes Music folder. I haven't tested this on a PC tho, I have separate user accounts, but don't run iTunes from the Windows boot on my Macs, anyone know if this works? Otherwise, you may need to copy your music into her iTunes folder under her account in Documents and Settings.
P.S. Her music is crappy, so I'm wasting a colossal amount of disk space by storing our Music libraries separately under user directories rather than using the shortcut (symbolic link) method to share libraries. We'll see if her music, or her goes if I ever need that space back!
