I found this in a few different places around the 'net - hopefully it's helpful:

Stop Windows Messenger From Signing In

If you're running Windows Messenger and it signs you in, even when you don't want it to, follow these steps to turn off signing in automatically:

• Click Start, click Control Panel, and then click User Accounts.
• On the Advanced tab, click Manage Passwords.
• In Store User Names and Passwords, select Passport.Net/*, and then click Remove. This removes your Passport?s connection to your computer, but doesn?t delete your Passport.
  
• Click Close, and then click OK.

When you restart Windows Messenger, it won?t sign you in automatically.