Interesting... here is a way I've just made up so if it doesn't work then sorry:
1) Open Script Editor in your Applications/AppleScript Folder
2) You are faced with an empty two-text-box-window...
3) Write in the TOP text box (the bigger box), I say this because mine reverts to the bottom one, where script cannot be entered.
4) type
display dialog "hello" replacing the "hello" with what you want to be said, for example "you smell"
5) Run program, see if it works
6) File -> Save AS
7) Save whereever you like, just as in Word or any other program, remember where you've saved it. Change file format to Application and click RUN ONLY not startup screen.
8) Your done...
9) Open System Prefs (Apple Menu or your dock)
10) Click Accounts
11) Click on the account which you want the message to pop up on
12) Click "Startup Items" tab
13) DRAG the script file you made into that window
14) Make sure "hide" isn't checked
15) Your done
Hope this has helped