Messenger by default saves your account password after login. This can be problematic if other users of the computer are able to login to the accounts that have their password stored and read your e-mail. To prevent this from happening in the future, make sure the "Sign me in automatically" checkbox is unchecked when logging in. This will stop the automatic sign in.
Use method two if you are using a Dutch version of Windows. The first and easiest method: click Start, then Run, in the box type: 'control userpasswords2' and click 'OK'. You will now get a screen called User Accounts, click on the Advanced tab. Now click the Manage Passwords button and delete all accounts of which you want to remove the password.
This one will delete all stored accounts from your computer and is a bit more tricky so you are advised only to use this with the Dutch version of Windows XP. Method one will not work due to a bug in the Dutch version.
Delete the Credentials maps that can be found at locations described below. If you have trouble locating them Windows is most likely set to not show hidden files. To overcome this open 'My Computer' from your desktop and click 'Tools', then 'Folder Options...', and go to the 'view' tab. Now look for 'Hidden files and folders' in the list and select 'Show hidden files and folders' and click 'OK'.
Finally, reboot your computer to see how it worked.