Actual harddrive must be replace at least each 2 years if they are used 10h/days, after that they lost 50% of the speed output.
My source : my main pecee made in 2001 (Sisoft Sandra 2003/2004, chipset HTP Raid 370, swap+temp on an another HDD)
IBM 46 Gb : after 1 year, around 40% of perforce lost, then brokedown
Maxtor 20 Gb : after 3 years, around 50 % of perforce lost
Seagate 120 gb : after 1 year, around 20 % of perforce lost
Maxtor 300 Gb : after 1 year, around 10 % of perforce lost
Capacitors can be also problematic.
I changed all of them on a 2004 WinFast Nforce2 (they explosed one after one...), and some on 2003 MSi Nforce2. They now both work like a charm, and performances are back.