My first hard drive (10 megabytes) was in an IBM PC, circa the mid eighties.  It failed after about 6 months of use.  Since then, I've had quite a few that have failed, including 2 Seagate 500 GB drives that failed recently within a month of each other.  They were less than a year old and enclosed in a case with more than adequate cooling.
Anything that you can't afford to lose should be backed up on CDs or DVDs or an off-premise server.  For me the D-Link DNS-323 is my 3rd line of defense against failure.