Both MSN and Yahoo mesenger programs allow for someone else to see when you come on line.... I thought it was from the sign on name but it could be linked to the IP and a computer IDentifier. Never really gave it much thought.

You might try clearing all your cookies and passwords and temp files to clear your system from selfsigning into the messenger service.

Google for ATF cleaner a free program that will clean out these items. Just select all and click ok to remove and it is done. You will have to re-login to all your websites and forums that you usually monitor....

Has this person had access to your computer with or without your permission? if so he may have installed a keylogger or some other spy program that you will need to search for to find.