The following are instructions for making sure Windows Messenger,
will not automatically sign you in.
I am running Windows XP Home.
I configured Windows Messenger on my system in the following way:
Set messenger to load when Windows starts.
Allow messenger to run in the background.
do not automatically sign me in.
With this configuration, the program sits in the system tray, until I need it, and doesn't get in my way
When I do open the program to use it, I now have the choice of which account I want to sign-in with.
This configuration seems to work fine on my system, and hasn't had any negative affects on Outlook Express.
The main reason for getting to this point is so no passwords are stored on my system.

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:
List of 4 items
1. Click the Start button, click Control Panel, and then click User Accounts.
2. Click the Advanced tab, and then click Manage Passwords.
3. 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.
4. Click Close, and then click OK.
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On Windows XP Home Edition:
List of 5 items
1. Click the Start button, click Control Panel, and then click User Accounts
2. Click the picture that is associated with your account.
3. Under Related Tasks, click Manage my network passwords.
4. In the Stored User Names and Passwords dialog box, click Passport.Net\*(Passport), and then click Remove.
5. Click OK, and then click Close.
list end