OK so I have had this on a number of desktops within the last few months. I dont know if it will work but it is defo worth a try, get to the BIOS of the machine and restore the factory defaul settings, if you can that is. Obviously I do not know enough about your current setup to know if this is possible but a piece of added hardware may be the root of the problem i.e. a SCSI card or ethernet card. Try starting it up with one of these items removed at a time e.g. take just the ethernet card out and try it, if no joy put it back and take the SCSI card out. I have seen machines die on me after having them for over 5 years simply because of a failing piece of hardware, it worked fine for years and then all of a sudden it is the cause of a system failure.
Just give it a go, cant hurt!