Depends why it is bad. Could have been there for years (a bad cluster could be from damage when the computer was jarred, and the head touched the platter), or the surface is starting to flake off. Keep back-ups of any needed data. Run defrag once a week, if it stays at one bad cluster, just use it as normal. When it starts showing more bad clusters when you run defrag, plan on replacement. You should also see more and more errors as the platter gets more bad spots.