There could be a whole load of reasons for this.
Is the PC still under warranty?
Do you have Everest home click here installed? This will give an indication of PC temperatures and fan speeds and may help diagnose the problem.
What exactly do the `error reports` say and what options do they give?
Are there any unusual noises?
Does it always shutdown after the same/similar period of time or when you are using the same software or doing the same job?
Have you carried out a scan with good anti-malware tools? What anti-malware tools have you installed and are they up to date?