To remove them from the sign in dialog, click the Start Button, then click the Control Panel.
Then, click User Accounts in the Control Panel. Select your account name, and then on the
left side of the screen, under Related Tasks, click 'Manage my Network Passwords'. In the
window that opens, you can remove accounts by clicking the passport you wish to delete
[denoted by (Passport)], and then click the 'Remove' button.