Basic rule of thumb for HDs: if there's any bad sectors its dying, and otherwise its fine. I've only ever had one hd die a premature death, and that was a Samsung 160 GB SATA that went bad after 3 months. Usually, with hard drives, you can get at least 3 years out of them before its time to start thinking about replacing it.
CD/DVD drives/burners are a whole other story for me... I don't think I've ever had one last longer than a year and my current LG dvd burner is already going flaky after only 9 months.
