try creating a new user account and logging out of the original one and in with the new one, and then seeing if expose freezes up on that account.
if so, i have absolutely no clue what to do, other than maybe an archive and install from the panther dvd.
that is a big extreme, but _hopefully_ either a. the new user account will work, or b. someone else can shed some light on this.
by the way, if the new user does work, then copy all the stuff out the old user account and use the new one.