Print screen a man desktop, hide all he icons, minimize the taskbar and watch him wonder why he clicking away at 'My Documents' or 'My Computer' wondering why it not working...
Alternatively, install the BSOD screen saver and unplug ever so slightly the keyboard and/or mouse (since the mouse can't wake it up but the keyboard can).
Also, use a BSOD wallpaper, hide all the icons on the desktop and disconnect the mouse....oh yeah minimize the taskbar.
Hmm.. what else... use VNC Viewer to remote a man machine and spam up he msn....
Switch the M and N keys on a keyboard...
hmm this list getting too long... I give away some of meh best too soon :p