My lattitude c600 boots up and lets me start working then turn to a blue screen "dumping physical memory" Then tries to re-boot itself but goes to a black screen and says " Hard disk read failure - no bootable devices". If I shut it off it reboots okay for a few minutes then repeats itself. Is it time to replace the hard drive? I bought it used and don't have the start up disks, how can I get them?