Anyway, Windows Error Reporting then opened a troubleshooting web page (http://wer.microsoft.com/responses/Response.aspx/6508/en-US/5.1.2600.2.00010100.3.0?SGD=86a6eb34-a6cc-44f5-bced-4f79815fd063&Bucket=0x8E_NPDRIVER%2b6036#here) which said "A problem related to your antivirus software caused your computer to shut down unexpectedly to protect itself from data corruption or loss.
To try to solve this problem, follow these steps. One step might solve the problem, but if it doesn't, then go on to the next step."
The three steps are:
1. Update your antivirus software (which in my case is MSE).
2. Check for multiple antivirus programs running on your computer.
3. Contact the antivirus manufacturer (which is Microsoft).
I checked the update status of MSE and everything is green - i.e. up to date & fully protected. I also did step 2 using the procedure the web page recommended and found no anti-malware services other than MSE. So here I am - step 3 - "Contact the antivirus manufacturer".
The only things that should have happened during the night are normal email checks and an MSE definitions update & quick scan. There was no new email, so it appears that MSE blew up. Does anyone know what might have caused this shutdown?