Do you have access to a friend's computer for ~1 minute + restart time?
Is your friend deserving of a practical joke?
This is a simple and effective April Fool's Day Prank.

Step 1: Downloading and Using KeyTweak

On the Victim's computer, download KeyTweak from (http://webpages.character.net/krumsick)
Install KeyTweak, following the onscreen instructions.

-Open KeyTweak.
-Select the key you wish to change, in this case, I chose Spacebar, which I will make into the Enter key.
-Chose the new action for the selected key under the "Choose New Remapping" menu.
-Click "Remap Key"
-Click "Apply"
-Restart computer as is prompted to do so.

Ta-da!

Step 2: Enjoy Watching the Victim Panic!

Having any key changed on your keyboard is a difficult thing to get used to.

These are some of my favorite remapping:

space :: Enter
Arrow Key :: make computer shut down
Enter :: Eject
m :: n && n:: n (They might think someone switched they actual keys on their keyboard and try to remove them and switch them back)
Disable a commonly used key, i.e. "r" (Instead of choosing a key to remap it, simply click "Disable Key."

Too Panicked?

If something goes terribly wrong, for instance the victim uses the specified key in their password to log on, you can always use the on-screen keyboard, provided by Windows, where each key acts as it always should.

Saving your Friendship.
If they cannot fix it themselves, or when the joke is over, open KeyTweak again and click "Restore All Defaults," restart once more and everybody's happy.