There are a few causes, most likely your hardware again.
Overheating is one cause...
Your hard disk might be dying. The chance of this happening is low.
To double check, plug it in to another machine. Make sure it's in
slave mode (if it's an IDE HDD). Boot up the PC and see if the PC
can detect this hard disk.
Also, if you are able to boot up, can you take a look at Event
Viewer (Start > Control Panel > Administrative Tools >
Event Viewer). Look out for any errors there.