MS bulletin on Event ID51 (does apply to XP)
and loads about it here
You clearly need to consider that the drive may be failing - so ensure that you are keeping up to date backups.
To monitor drive performance you can load a SMART monitor, which will tell you if any drive parameters are out of kilter.
Google hard drive smart monitor for a variety of such programs.
I would be inclined to try a new data cable, too.
What does 80% healthy on spin-up mean in the diagnostic you ran? Should it read 100% if there are no problems?