|This usually refers to other local users. Go to Control Panel /Users and get rid of those you don't want.|
But it would be impossible for local users to log on without your knowledge. Check your start-up programs (Start/Run/msconfig/startup tab) to make sure some remote control program isn't starting automatically. Look in Add/Remove programs. Do a deep malware scan. Also run HijackThis. With a port scanner you can view your active ports as well and look for unauthorized activity.