Mac OSX font display

Hi, I have a Mac Mini running Osx 10.4, tiger.
It has run fine for a long time, but suddenly it has stopped displaying the fonts on the icons, settings menus, file explorer etc.
Instead of the correct characters, it displays a boxed capital A for each one.
When slow double clicking on the icon to rename, the correct characters reappear, and can be edited, then when the renaming box is exited, the A symbol reappears again.
Most annoying is i cant even check settings, as all i can see are rows of A symbols!
Any help would be much appreciated,
Thanks