Thanks for the replies.
Chiron, I downloaded a fresh copy of the Firewall, diconnected from the web and disabled both Avira and Windows Firewall. I forgot to mention before, I also had Spybot installed. I completely uninstalled it, before trying to install Comodo again. With everything disabled or removed, I still got the bluescreen and shutdown, several times. The error message says basically, "An unknown error has been detected and your computer is being shut down to prevent it from being damaged."
EricJH, I installed Bluescreenview and have some information about the 3 crashes that occurred today. It doesn't seem to have a record of the two shutdowns that happened several days ago. What's interesting is, the first two crashes were caused by driver, msahci.sys, but the third shutdown was caused by a different driver, ntoskrnl.exe.
Several months ago, my PC was infected with the "FBI scam" virus. I don't know its real name, but apparently it's been very widespread in the last year or so, and seems to be a pretty nasty virus. Things got so bad that, several weeks ago, I was forced to install a fresh version of Windows 7 on my computer. The original version of Windows 7 is still in the PC, cached in the directory, windows.old. Is it possible this problem is being caused by a residual effect of the virus? Since installing the new OS, when Avira scans my computer, it always finds several "hidden files," all of them contained in the windows.old directory.
I would like to ask, since I have no special need to keep a stored copy of an old operating system, is it possible to delete the entire contents of the windows.old directory, and remove any malware that might be hiding in it? All help is appreciated.