All file, menu, programe names replace with symbol [A] ...help ?

[beastie_boy]

Hi everyone,
Switched my G4 mac book on yesterday and all file names, menu names and programe names have been replaced with the symbol of an 'A' in a square. Making doing any thing on it near impossible,
The symbol looks a little like this [A] and there is one for each letter of a name that is has replaced.

Any help ?
thanks in advance.
Andy