That being said, here's exactly what I'm going to show you how to do:
1. Create a message that is viewed before a person logs onto their computer (saying "April Fools... etc."). (This can be done on any version of Windows)
2. Change the text on the start button (since XP was the last version to say "Start", this will not work on later versions).
Step 1: Create a Legal Message (AKA Logon Message)
Once you are in the registry, be sure and backup the registry by clicking File-Export. Save it as whatever file you want to.
Step 2: Modify the Registry
Find the entry that says "legalnoticecaption", and double click that, and change the value data to "Happy April Fools!", and press "OK".
Then find the entry that says "legalnoticetext", and double click that, and change that value data to whatever you want your text to say.
If you don't want to change the start button text, just shut down the computer now, otherwise, keep reading.
Step 3: Download Resource Hacker
Once you have installed it, start it up. Click "File-Open", and navigate to "C:\Windows\explorer.exe" and open that file.
Step 4: Change the Text
Change the text next to 578, from "start" to "whatever you want here", then click "Compile Script". Keep the text short and sweet... I've heard that there could be a character limit on some versions of XP, although I've never run into that myself. Then click "File-Save As". DO NOT CLICK SAVE OR SAVE THIS AS THE SAME FILE NAME!!!!!!! Save your file as explorer2.exe.
Step 5: Modify the Registry
Now we are going to navigate to "HKEY_LOCAL_MACHIENE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon. Modify the "Shell" entry to "explorer2.exe".
Navigate to "C:\Windows", and find the file called "explorer2.exe", and double click on it. If it worked properly, it should open up a folder. In many cases it may not work right on the first try and it will give you a message saying it isn't a valid application. Just open resource hacker up again and do step 4 again. Most times it will work on the second try. Then restart your computer to be sure everything works as it should. If it does, just shut down the computer and wait for the fun to start. If it doesn't work for some reason, see step 6 on how to fix the computer's registry.
Step 6: How to Fix Your Computer If There's a Problem
Go back to task manager, and start a new process "explorer.exe". The computer should now be back to normal, except for the legal/logon message. If you want to remove that message, just go back to "HKEY_LOCAL_MACHIENE-Software-Microsoft-Windows-CurrentVersion-policies-system" and clear the value data for the legalnoticecaption and legalnoticetext.
Good luck, and happy pranking! :)