There aren't many occasions when I can honestly recommending using the Dr.Windows practical joke program I first released in 2005. But April Fool's day is one of them.
The basic idea of this program is that it sits in the system tray of your victim and can be configured to occasionally and randomly pop up humorous or non-sensical error messages that look exactly like normal windows error messages, except for the content:
Now keep in mind this is just a bit of harmless fun, and the program won't make any real attempt to hide itself from your victim -- it's icon is shown down in the tray after all -- we wouldn't want to really freak someone out now would we?
Because of the fear this year about the conficker virus, i strongly recommend that you remain present while your "friend" discovers this little prank, just to make sure that they don't think it's a virus and freak out.
In fact -- the smartest and easiest thing to do is to install Dr.Windows on *your* pc, and then you can bring it up on demand by surreptitiously hitting the hotkey (ctrl+shift+E by default but you can change it), and then call your significant other or your IT worker over and ask them to explain why you are getting these inexplicable errors..
And for that extra bit of fun, you can create your own custom error messages.. Just try not to be too evil, please.