In that case, what you need to do is create (or have your partner create) a separate user ID for you on the computer you will be using. Log in as yourself, and copy the backup of your iTunes library to the appropriate place in your user profile. Check iTunes to make sure everything is there, and set up the phone to sync. Any media will be erased from the phone and replaced with media in what will now be YOUR iTunes library. You will have to be careful (as will your partner) to make sure the correct person is logged into Windows BEFORE attempting to sync or you both risk accidentally wiping your phones and syncing data you don't want.
It was not apparent from your initial post that there had been a problem with your computer. It appeared on first read to imply that you simply wanted to get media from someone else's machine onto your phone.