Thanks for the help, now sorted.
Managed to get the font from my laptop and transfer it to my desktop, but why it disappeared in the first place is a mystery.
I'm assuming fonts reside in the basic windows xp operating system and are then used by various programs.
Basically went control panel-appearance and themes-font(in left pane).
This lists all fonts in the operating system and posterbodoni was missing, so transferrred font from lappy.
Thanks folks.