Keylocker is a pair of programs for Windows allowing administrators to dissallow input to a remote PC. There is a small client and a control program.
NEWS (28/06/2006): I have just done a quick test in Vista, and apart from it being slightly more annoying with the firewall, Keylocker2 still works fine.
The files are now up at the project page.
To comment or get help, join the mailing list which is also available on the project page.