You need separate Windows accounts to login to from the Welcome screen. Then you have your own everything (except the iTunes program itself, there's only one of that). Libraries and everything else is private, however, limited to each Windows account.
Go to Control Panel>User accounts and make a new account for her.
Our home PC has my iphone account already set up on it. My daughter has now purchased an iphone and wants to be able to buy apps, music etc and sync from the same computer but we want entirely separate accounts as our contacts, apps, music, photos etc will be different.
I thought I had carefully set up her account separately with a new name and a new password, different email and contact details etc but when we plug in her phone the system still automatically take us to my account and asks for my password and won't allow us to click on anything to swap to her account or give us any similar 'either/or' option.
What have I done wrong and how can I fix this so our phones are treated entirely separately on the same computer.