Thanks for the thoughts.
In all fairness I realize that my Subject should have been Computer freezes during Boot-Time Scan.
The keyboard is unresponsive. Pressing ESC does nothing.
Initially, I did get the screen showing that the Boot-Time Scan was running. (I made sure that Avast was scanning through the files before I left.) However, when I returned a few hours later the monitor was blank. I can not say whether the option screen came up and then it froze or whether it froze before the option screen appeared.
I have no other anti-malware programs running during the boot up.
I do have a considerable amount of data on my 300 gig drive. However, normally, when I run Avast, the HDD indicator light flickers. The indicator is now on solid. It has now been over six hours since the scan began.
I was getting some kind of updater PUP (I forget the name) that kept coming back after two other Boot-Time Scans (the first log had three instances and the second log had one instance) so I decided to run it again and see if the log came back clean.
My two questions remain: 1) Will I hurt anything if I power off the computer in the middle of an Avast boot-time scan before the options screen appears? 2) What could cause Avast boot-time scan to freeze?