It depends on your SMART HDD analyze.
Almost all hard drives these days has limit of the bad sectors (it's called Reallocated Sector Count) which can easily handle, but if your bad sectors reach this limit and still counting (you have to observe this number regularly)... it will be better to start lookning for the new drive since your old one can at any time not start again.
You can install freeware HD Tune
(v2.55) to check SMART of your HDD (Health Tab).
Please note that if it is a hardware issue with your hard drive then reformat doesn't help.