From John Fitzgerald at Shootyourself.net:

“Trashing Prefs” is an absolute must when you are using FCP. From time to time, FCP will start behaving in odd ways – error messages, system crashes, failure to recognize firewire devices, etc. You name it, it has probably happened to someone at some point. Chances are, “trashing prefs” was the solution to a majority of the problems.

Read the complete tutorial here.