This is a PC prank, to make a Windows box appear to be locked up - no response from the Start Bar, no response from the icons on the desktop, nothing. To someone unfamiliar with keyboard shortcuts (more on this later), the PC will appear to be completely locked up. Rebooting will not fix it, either.
While there are a few steps involved, once you have practiced them, you can get it set into place in just the space of a co-worker's walk to the printer & back.
Step 1: Create a Picture of the Desktop & Set It As the Wallpaper
The first thing you must do is to create a wallpaper of the current desktop - this picture includes the current wallpaper image, as well as the start bar, and any and all icons on the desktop. You must close ALL applications, and not be running ANYTHING.
Once you've done that, press the "Print Screen" key. On some keyboards, it is labeled PrtScn or something similar. Pressing that puts an image of the screen in the clipboard. launch Microsoft paint, choose Edit/Paste, and you should now see an image of the entire desktop. Save the image onto the hard drive, then choose File/Set as background (Centered)
Step 2: Hide All Desktop Icons
On the desktop, right-click and choose "Arrange Icons By...", then UNcheck the option "Show Desktop Icons". You can verify you've done this by trying to move an icon. You should not be able to, since it is part of the wallpaper image (the real icon is hidden).
Step 3: Move the Taskbar
Move the taskbar by dragging it to another part of the screen, (I like the left or right side of the screen, personally). You may have to right-click on the bar itself, and UNcheck "Lock the Taskbar", THEN you can drag it to the left or right side of the screen. It will look a little weird at this point, since you have the actual taskbar, as well as the one that is part of the wallpaper from earlier.
Step 4: Hide the Start Bar
To hide the taskbar, right-click on it, and choose "Properties". Make sure there is a checkmark in the box by "Auto-hide the taskbar" At this point, the stage is set!
Step 5: What to Do Once You've Done the Prank
Once you've done all the steps up to this point, how you handle your victim is up to you. Whether you immediately let them in on the joke, or draw it out, or even tell them at all what you've done is up to you! If it's your mortal enemy, do nothing; let them suffer computer withdrawl, since their stubborn pride will no doubt prevent them from seeking knowledgeable help (i.e.: You) to fix this dastardly problem.
If it's an officemate, or your sibling, or parent, and YOU are their go-to computer guy, again do nothing, as they will come to you. What you do when they come to you depends on how long you want to draw out the joke (and whether or not you can keep a straight face when you do so!). One of my standard answers when I see something broken is to wag my finger at them, and say that they need to stop visiting THOSE KINDS of web sites.
Step 6: Keyboard Shortcuts
Okay, here's the section on keyboard shortcuts that may be of some help to you when you pulling this prank on someone. See that little Windows logo key on either side of the keyboard? Yeah, that one. Well, it's actually a very useful key, if you like keyboard shortcuts. The Windows key can be pressed by itself to bring up the Start menu (same as if you clicked "Start" on the taskbar. It can also be used as a key-modifier, to do different things: Windows-E will bring up "My Computer" ("e" for "explorer", as in computer explorer), Windows-R will bring up the Run dialog, Windows-F will bring up the search dialog ("find"). Windows-M will minimize all active windows, or restore them if you press it again. In Windows 7 there is a number of new options, involving the up, down, left & right arrows when used in conjunction with the Windows key. You can do a google search on "Windows key combinations" - This wikipedia page: http://en.wikipedia.org/wiki/Windows_key seems to be cover the subject well.
My point in bringing this up? Well, if your mark comes to you complaining that their computer is locked up, after you expound How odd it is that the mouse is working perfectly, and why won't anything respond?!?!", while you are frantically clicking on My Computer on the desktop, with your other hand, you can click Windows-E, and the Computer Explorer will come up, just like magic! You can then click into the My programs folder, and launch a program by clicking directly on the executable, and proclaim the problem fixed! Or expound on how odd it is that the program runs when you click on it, but why won't the Start button work?!
One caveat to the previous steps might be to NOT move the taskbar to the left or right side of the screen, just leave it in the location it was in. That way, with the taskbar at the bottom of the screen, just hidden, you could loudly double click on the start button (which, remember, isn't actually there), and press the Windows logo key, and the start menu will come up. You can then explain that it appears to be a faulty mouse, and they just weren't trying hard enough. You then ask them to try it, and when it won't work, you sit in front of the computer, and try it, slyly hitting the windows logo key while distracting them.
it all depends on how you want to run the scam. Whether you shout "ha-ha! Got you!" almost immediately, or whether you convince them that you need to take their box away & rebuild it, then return it to them a week later, proudly boasting that you were able to save all their documents, programs & everything, "Why, it doesn't look like it changed a bit, does it?"
if you've read this far, you have made it to the end of my first instructable; hope you enjoyed it.Thanks!