Just a few questions first...
How old is the HD?
What size is your HD?
Have you defragged/scandisked it?
What have you got running in the background?
Do you have any scheduled tasks setup eg updates or virus scans?
Just need a bit more background info. If your hard drive is grinding all the time, it's either damaged, fragmented or the system is not paging files correctly.
There could be a few explanations, so give as much detail as possible for a more accurate remedy.