Checking hard disk with fsck

[Anthony Papillion]

As some of you know, I've been having a bit of problems with my video 
blanking out and my system freezing at weird, random times. I thought it 
might be a video driver problem but I'm now convinced, after a bit of 
poking around, that it might actually be a hard disk going bad.

So I was going to check the disk using fsck but, when I tried, it warned 
me that the filesystem was already mounted and that 'SEVERE' damage 
would happen if I continued. Obviously, I don't want that to happen so 
I'm asking you all: how do I properly check a hard disk for errors?

Thanks,
Anthony