Bad sectors are areas in your HDD which are damaged.
If you want to check the extent of bad sectors, try running Scandisk either from Windows or from DOS. Incase if you arn't able to boot to Windows, start your computer with a DOS bootable and type the command Scandisk. This will first of all run a quick scan and then will ask you for a surface test. Opt for Yes in this and it will then show you all the bad sectors in your HDD. This procedure will also help you by marking all the bad sectors as bad so that further no data will be written in those areas.
Data at risk? Ofcourse it is. All the data which is stored in the bad sectors wouldn't be recoverable any more. You are highly recommended to back up all your critical data as there is a possibility of more data loss.
Ofcourse there are a few tools available, but none of them can gurantee you a 100% repair. The best way to get rid of this is to repartition your HDD and then do a low level formatting, but mind you, all your data will be lost in this procedure, so backup every thing before going in for this. Moreover, this is a bit technical procedure. So incase if you are not aware of the procedure, dont do it yourself. Take the help of someone who knows about this.
You never mention your OS, capacity of your HDD and it's make. Do add it up to get better results.
