Section 1: My hard drive has bad sectors / Blocks / area
Do i need to change it ?
Not necessarily, but If it is in warranty, and they allow you to replace it, a new one is not a bad idea, otherwise read on
it all depends on whether the bad sectors are expanding or not, if they are not, they are probably caused by shock to the hard drive, usually, it is enough to mark them as bad using “chkdsk /r” on windows and leave the drive working.
To find out if your bad sectors are Spreading or not spreading, do a “chkdsk /r” four times, make sure the same number appears in the second and third and fourth time (Forget the first time), then, if the second is different but the third and fourth are the same, then do the test 2 more times, and make sure you get the same number of bad sectors for trials 3, 4, 5, 6, if so, your bad sectors are not spreading.
You did not mention backup in the answer before, do we need to backup ?
People would typically ask you to backup just in case, i say you should always have backup of your most important files, non spreading sectors of the hard drive, in my humble experience do not contribute negatively to reliability, so my answer is, backup should be done regardless
How do i know how many bad sectors are marked on an NTFS hard drive ?
There is a tool called nfi.exe that comes with a bundle Microsoft makes available here http://support.microsoft.com/kb/253066/en-us this tool is part of (OEM Support Tools), it can tell you everything about a disk formatted in NTFS