WARNING:
If you have just one drive and it's making unusual ticking sounds or you get a SMART error on boot, turn off your computer and don't use it at all until you're ready to attempt making a copy of important files (i.e. family photos you have no copies of).
A couple of notes for those who might not know:
SMART errors are not always a SURE sign of hard disk failure. A hard disk can sometimes run for some time with a SMART error, but replacement is still VERY highly recommended. You can choose to turn SMART off in most BIOS's today, although it's not typically a good idea. Still; if you're broke, know about your bad HDD, are tired of the error warnings on boot, and don't any have important data saved on your computer that you mind losing...
Western Digital has Datalife tools as well. All the big HDD makers have a utility to look at hard drive S.M.A.R.T. status. Many SMART errors are warranted. The drive doesn't actually have to stop working completely in order to get a warranty replacement, but check the warranty to make sure your SMART error is covered by the warranty. Some manufacturers will send you a replacement disk with a hold on a cash amount in your bank account or on your credit card which they will refund if they recieve the bad drive from you within a given time. Just make sure you can live without the cash (and have it in your account if using a checkcard) during that time. WD surprised me a little on this and I got charged overdraft. :-(
BTW, this ought to be a STICKY.