How to repair bad sectors of a hard disk?

I have two Hard Disk Drives.
1>Seagate 80 GB
2>Hitachi 40 GB
I think bad sectors are formed on my seagare gard disk.
Everytime I start my Pc, Checkdisk repairs the files of “D” Drive.Also some files are lost.The computer frequently hangs and has become slow.How to correct this problem?

It is Not Possible to Repair Bad sector on hard disk drive because bad sector’s mean the scratches on the plater HDD consist of plate circular disk called as plater. plater are inside of hard disk drive.
when we are regularly formate the HDD then the bad sectors are created on a hard disk