desktop shuts down or goes to black--hard drive failing?

My 5 1/2 year old HP Pavilion desktop has been going to black or shutting down and restarting intermittently. I have done HP diagnostics and they showed no problem. I did a restore to an earlier date a couple of days ago, and a few months ago, a complete system restore to factory settings. The shutting down and black screen occurred after the restore. I am wondering if these are symptoms of an imminent hard drive failure. Or, does it sound like another problem? I have not added any new software or hardware. I have done regular windows updates. I am running Vista.
Thanks!