Microsoft Word

Here is one written in Word 6 but should work in any version, 
it displays "Hello World!" in every font you've got, then repeats forever.

To run it...

1) Copy it to the clipboard
2) Start Word if not already running 
3) Use Tools then Macro and select a new name you like.
4) Paste the text into the macro, between Sub Main & End Sub lines
5) Run it!
It will run endlessly (press escape to interrupt it).

Macro text follows:

Sub MAIN
REM open a new document
FileNewDefault
REM Insert the text
Insert "Hello World!"
REM Center it
FormatParagraph .Alignment = 1
REM Loop forever
While (1 = 1)
REM Find how many fonts user has installed 
REM and loop through them
For count = 1 To CountFonts()
REM Select the text
EditSelectAll
REM Format in next sequential font 
Font(Font$(count)), 40
Next
Wend
End Sub

End of macro text...