A bad block is a damaged area of the disc which cannot be written to. You're presumably talking about a CD or DVD you're trying to burn? If that's the case, chuck it away and use another one.
If on the other hand, you're trying to copy a CD/DVD to the hard disk, then do the following.
- Open Windows Explorer and then right click your C: drive and go to "Properties".
- Click the "Tools" tab and then in the top menu called "Error checking", click "Check Now".
- In the next menu which appears, checkmark both boxes and then click "Start". You'll get a message that the tool cannot run because the drive cannot be locked and whether you want to schedule a check on the next reboot. Click Yes and then reboot.
Chkdsk will run and mark bad sectors a.k.a. "bad blocks", so that they can't be used. Be aware though that checking for bad sectors can take several hours, so only do this when you're not going to be using the PC for a while.