If your PC is likely to be used or interfered with by others, and you want them to keep their sticky paws off it, here’s a way to create a special message that will appear when anyone other than you tries to logon.
In Vista go Click Start, All Programs, Administrative Tools and Local Security Policy. XP users should go to Start > Control Panel > Administrative Tools and Local Security Policy. In both cases expand Security Options and in the Details pane double-click on Interactive Logon: Message Test for Users Attempting to Logon’. In the box that appears compose your message; make it pithy, insightful, or just plain threatening, click OK and after a reboot it’s ready to run.