Hacker Attack on NT Server

I’m on a project. Three laptops. running Windows NT Server and Windows 2000 server are workgrouped with a cisco router. One of my coworkers keeps hacking onto my laptop and loading annoying screen savers, placing programs on my startup and other generally annoying stuff (nice guy – huh?). I have asked him to stop but he thinks he is being a great wit. I don’t want to start a hacker war with this idiot…just want him to stop annoying me. Any suggestions on how I can prevent him from hacking onto my laptop would be appreciated.