This is a simple prank to carry out on WinXP machines. It uses the 'display web site' functionality on the desktop to show a funny flash video loop of a pug licking the inside of your PC screen.
Step 1: Get into your victims Desktop settings
I have only tried this on Win XP, not sure if Vista has this function. Have a try and let me know.
1. Right click on the desktop of your victims PC. Then select properties to bring up the desktop settings.
2. Click on the desktop tab
3. Click on the 'Customise Desktop' button