Grey shutdown screen

Have been experiencing the grey shutdown screen at least 10 times during the past 3 days. Searched the forum and some have been experiencing it on some older models. My specs are:

• 10.7.3
• 2.4 GHz Intel Core 2 Duo
• 8 GB RAM
• 500 GB hybrid drive

I have done the following:

• Repaired disk permissions
• Verified the disk
• Run the extended Apple Hardware Test with no problems detected

Does anyone have any advice? Could it be a software issue? I finally got Windows 7 Ultimate N to run in Virtual Box. However, the shutdown happens at all different kinds of uses.

Too little info to know if it's software or hardware.
Time to start looking at the details of the kernel panic, and read the logs using Console Utility, to see if you find hints there.

I am still new to Mac....how does one do that?