iTunes 10 for Windows: Sign in or out of the iTunes Store
After you set up an Apple ID account (or if you’re using an existing AOL account instead), you can sign in to the iTunes Store to buy music and video, view or change your account information, and see your purchase history.
When you finish using the store, you can sign out to prevent anyone else who has access to your computer from making purchases using your account.
To sign in to or out of the iTunes Store:
In iTunes, choose Store > Sign In (or Sign Out).
You can also sign in to your account by selecting Sign In from the navigation bar at the top of the iTunes Store window: