There are two ways of doing this, depending on whether you are logged on with an account that has administrative privileges or not.
Steps with Administrative privileges account:
1. Go to Control Panel >> User Accounts.
2. Within “User Accounts” double click your account name.
3. This will open a new window which on the left, in the “Related Tasks” section, click on “Manage My Network Passwords.”
4. This will open the “Stored User Names and Passwords” window which shows the list of MSN Messenger user sign-in names. Select the names you wish to remove and then click the Remove button.
Steps using Limited privileges account:
Repeat steps above except for step 2. The difference here being that by selecting Control Panel >> User Accounts takes you straight to your account details “ONLY.”