cannot read boot file

I just ran Norton Antivirus and this error: I get the same error for drive D. It reads the Master boot(s) just fine.
previously tonite the computer was a bit sluggish, had difficulty closing programs, difficulty rebooting, getting to pop up. I ran MSscandisk then MSdefrag then Norton A/V and got this error.
What is wrong, how do I fix it?
I am a bit of a novice so if you have suggestions, please be detailedwith your instructions.
Thanks for your help
murphy