Q: What is S.M.A.R.T. and how does it affect my PC?
A: S.M.A.R.T. stands for Self-Monitoring, Analysis, and Reporting Technology. It's basically a self-diagnostic for your hard drive. As it runs, a tiny CPU inside monitors certain parameters, and if they fall below a certain threshold, triggers a SMART error. With compatible BIOS and/or IDE controller, you will then see the error, which will allow you to do something about it rather than experience sudden hardware failure.

Almost all hard drives nowadays are equipped with SMART. If you want to see what kind of values your HD is putting out, download Passmark's DiskCheckup and take a look. It's free for personal use, and it's read-only.

P.S. if you do see a S.M.A.R.T. error, your hard drive is DYING. There is nothing to FIX. Save the data, and replace the hard drive.