Unexpected shutdowns with no log or event, how to debug

[wmac]

Hi

My PC suddenly started these strange sudden shutdowns yesterday.

Windows 7 (32) shuts down unexpectedly sometimes after 1-2 hour and sometimes earlier. Initially I thought it might be related to the external hard drive I bought recently (because windows would not start if I disconnected it). However I connected the drive, brought up Windows 7 and safely disconnected drive and restarted.

The shutdowns do not produce Minidump, any event or log. What are my options to debug the system?

BTW Windows 8 installation on the same PC works with no problem.

Thanks.

 

[TrainableMan]

Are you sure it isn't going to sleep or hibernating? Sometimes they just don't wake up properly. So are you sitting there using it when it shuts down or are you away for a while? Also is this a laoptop or desktop?