You running Windows XP? I know this may sound crazy, but they myay have access to your system using the Remote Desktop Connection... That would allow them to watch (and interact with) everything you do. I know I played a few pranks in high school using similar remote access applications (for instance, I would be chatting with someone on AIM, and respond to questions before they hit enter to send the message, or I would respond to things that they typed but deleted). You should do some research (online for programs that have the capability and on your local machine to look for clients to such programs) to ensure that someone isn't accessing your system through such a program.

AJ