Well, a Sector is 512 bytes (on older drives), so each GB of capacity uses 2048 Sectors. If you have a 100 GB HDD, it has 2,048,000 (approximately) sectors in use, and 2636 bad - that's about 0.13%. If your HDD is 500 GB, you have about 0.05% bad sectors. Does not sound bad to me.
If you want to be sure, go to the website of the manufacturer of your HDD and download their disk diasgnotic utility package. For example, Seagate's is SeaTools, WD's is Data Lifeguard. Use their first tools to do two things:
1. Check what it gets from SMART alarm reports from the HDD.
2. Run the Long Test of the HDD. It does NOT damage the data it contains, but it will take a long time to test every sector. This is a different test from what Windows does in CHKDSK.