I go through my normal routine - check for duplicate devices in safe mode, look for known offensive or problem programs and drivers, and do a burn in overnight. Failing that, I then inform the customer that I couldn't reproduce the error, and that if they can't give me a specific instance of when it locks up, I'm gonna have to format and re-install. If they refuse, I make sure they read the disclaimer at the bottom of our shop tickets - in a nutshell it says that by signing this, you understand and are satisfied with the diagnosis/repairs done on your machine. That way, they can't come whining to me a week later that their computer is still locking up.