hard drive slow and with strange behavior

I think my hard drive is failing because after a series of tests with Disk Utily and Apple Hardware Utility, I got errors. For example, Apple Hardware Utility test has detected an error 4HDD/11/4000000 SATA(0,0). My Mac is slow and with strange behaviour.

I think I will replace the drive, but some people said that perhaps the HD fan is with problem. But when I run iStat Pro it shows the HD fan running at 1200 rpm.

How could I be certain that the problem is only with the hard drive and not the fan?

I found the procedure to replace the fan in this site, but you don't sell this part. In case I need, where can I find the HDD fan?