is it possible for someone who posts in the same forum as you, to scratch or do damage to your hard disk?
if they are a computer whiz? and if so, how do they do it (i don't want step by step, just an overview)
I don't think so.
They can get your IP address and trace that back to your machine. then delete files or place viruses. but they cannot physically damage your hard drive.
I am not 100% sure if they can totally mess up ur harddrive physically. But they can make ur harddrive spin over and over, although they would of had to have sent you some kind of server/virus/trojan.