Another possibility we must consider is a failed HDD. If you have access to another machine capable of burning a CD, download Seatools for DOS ISO (http://www.seagate.com/support/internal-hard-drives/consumer-electronics/ld25-series/seatools-dos-master/) and burn it to a disk. Change the boot order in BIOS to DVD drive 1st and run the software, start with Short test and if it passes that try the Long one...