If you mean in a way to stop people using the computer system, why not just remove the keyboard and keep it somewhere else?
It would be easier still if you had a wireless keyboard and mouse and removed them both - a mouse could be used to operate the system without the keyboard (don't forget, for instance, that you'll find a keyboard replica in Accessories>Accessibility).
Keyboard Shortcuts Result in Windows ® XP - General
Windows Logo Display or hide the Start menu
Windows Logo+BREAK Display the System Properties dialog box
Windows Logo+D Display the desktop
Windows Logo+M Minimize all of the windows
Windows Logo+SHIFT+M Restore the minimized windows
Windows Logo+E Open My Computer
Windows Logo+F Search for a file or a folder
CTRL+Windows Logo+F Search for computers
Windows Logo+F1 Display Windows Help
Windows Logo+ L Lock the keyboard
Windows Logo+R Open the Run dialog box
Windows Logo+U Open Utility Manager
So it's supposed to be Windows Logo + L to lock your KB, but all that does on MY PC is take me to the Windows log on screen. To re-enter I have to click my user name and enter my pass, but I don't use a pass! (XP Pro)