From the NYT tech section
Hard drives spend their waking hours spinning constantly and contain several moving parts. And as with most motorized mechanical devices, the moving parts can wear out or break down, causing the drive’s performance to decline. There is an adage to the effect that “all hard drives will fail eventually,” but depending on how long you use the drive, “eventually” may be long after you have upgraded and moved on to a new drive.
The signs of a hard drive in decline can include grinding or clicking noises, frequent crashes or other erratic behavior. (The fan that cools the computer’s power supply can also make a buzzing noise, so you may need to listen carefully to pinpoint the source of the noise.)
Hard-drive performance may also seem slow or sluggish if the drive is packed full of data or is in need of reformatting. At www.microsoft.com/atwork/getstarted/speed.mspx, Microsoft has some tips on increasing the speed of a hard drive, using utility programs that are included free with Windows. Mac OS X includes its own Disk Utility program that can fix certain issues that may be causing odd hard drive behavior, and there are many third-party disk-repair programs for both Windows and Mac systems.