This is from the Help file in MSN - seems to be what you want to do.
Searched for login - 2nd topic "Delete your Microsoft .NET Passport information"
To delete your Microsoft .NET Passport information from your computer
When you click the Sign me in automatically check box when signing in to .NET Messenger Service, your .NET Passport name and password are stored on your computer. This allows Windows Messenger to sign you in automatically. You can delete this information to prevent being signed in automatically.
On Windows XP Professional:
- Click the Start button, click Control Panel, and then click User Accounts.
- Click the Advanced tab, and then click Manage Passwords.
- In Stored User Names and Passwords, click Passport.Net\*, and then click Remove.
- This does not delete your .NET Passport, but it prevents automatic sign-ins.
- Click Close, and then click OK.
On Windows XP Home Edition:
- Click the Start button, click Control Panel, and then click User Accounts
- Click the picture that is associated with your account.
- Under Related Tasks, click Manage my network passwords.
- In the Stored User Names and Passwords dialog box, click Passport.Net\*(Passport), and then click Remove.
- Click OK, and then click Close.