I have a Sony Vaio PCG-V505ECP laptop. It is a little over 2 years old. Six months after I got it I got this exact message (Operating system not found). I contact Sony support and they had me send the laptop back. When I got it back the information indicated that they had replaced the motherboard, hard drive, and the processor. The unit ran fine for about 18 months and now I've got the message again. The unit gets very light use by the General Manager here (mostly he watches DVD movies while traveling). I am a Network Administrator and work on PCs regularly. I've tried several things including a bootup disk that I use and it indicates that there is no disk in it. I took it apart and made certain everything was tightly in place, including the processor and the memory but nothing has changed. If it is the motherboard again, I priced it at $1036.00 which is more than the unit is worth so I was hoping someone there would have some ideas that I haven't tried as yet.