Ditto. If they're really bad sectors it likely means you've suffered a head crash. In modern drives it's fatal - the damage will get worse as debris from the first head crash floats around inside and causes a domino effect of more head crashes.
Backup you data IMMEDIATELY.
Download the HDD manufacturer's diagnostic and run the thorough (might be called Advanced) test. Might take an hour or two. If it fails, it's a bad drive.
If you catch this early enough before any more damage occurs you could clone the old drive contents to the new drive. Unless you have a mini IDE interface adapter lying around, and a copy of Norton Ghost, you'll need to take it to a technician to do this. Take it to a small Mom & Pop shop (DO NOT take it to a large retail chain store!!! You've been warned).
If you don't mind re-installing the OS then just swap the drive and do the re-install.
Hope this helps...