You can do this with Plus! right through your contact list. Right click on the contact name, go to Messenger Plus! Extras > Notifications. Select "Notify me each time" and "Contact's status changes to" and set that as online. That should do the job.
![:)](images/smilies/msn_happy.gif)
All you have to do is make sure you have Messenger Plus! and the compatable version of MSN Messenger.