Some bad sectors are quite normal. The main thing is to monitor the drive and see if the problem gets worse.
Important!**Start backing up Critical Files etc ( if you have not already done so )**
Go to the website for the drive's manufacturer and get a drive diagnostic utility to diagnose your drive.
Bad sectors can be created by software errors, or slight errors in the read/write heads which cause the data laid down on that particular sector to be completely illegible, and thus appear as bad to the operating system. This type of 'soft' bad sector can generally be fixed by completely erasing the disk (by writing every sector over with a pattern of zeroes).
Bad sectors created by a mechanical error have a tendency to breed more bad sectors quickly. Since the margin for error inside a working disk is so slim, any surface damage or debris created by a mechanical failure or 'head-crash' will quickly cause more damage as it interferes with the read/write heads. When formatted, a drive with 'hard' bad sectors like these will use the spare sectors to as much as possible replace the existing bad sectors as detailed above. Of course, if the bad sectors are still spreading due to loose particles or surface errors in the disk, the disk will still be unstable. At this point, the only option is to salvage as much data as possible from the drive before it dies completely.
http://www.pcstats.com/articleview.cfm?articleid=1583&page=4
If the drive is under warranty you may need to get it replaced if the problem gets worse.