Bad sectors cant be "repaired". Hard disks use something called "sector sparing" which means they swap out the bad sectors with a good ones from a pool of spare good ones. Obviously there are a limited no. of reduntant sectors so after a while if the no. of bad sectors becomes too much, there wont be any good ones left to swap. Thats when the hard disk will go belly up.
Keep a watch on the Reallocation sector count (no. of bad sectors)- if its stable, its OK. If there are sudden sharp rises best to back up your data and be prepared to shell out for a new disk.