Check how defragmented the hard drive is. I recommend AusLogics Defrag. If it's above 10%, defragment it. See if that helps.
Thanks for the advice
I will try to defrag it.FYI, I have tried error-checking for the hardisk.But it takes almost whole day to finish just only 100GB (coz o've partitioned it).Does that means anything to u?Or I should change to a new hardisk :P