PC reboots after iAVS update is downloaded but before it actually gets installed

Lately, every time a virus update is downloaded and starts to get installed a a PC with Win2K SP4 reboots itself.

This just started happening after the latest program update (4.8.1368) was installed on or about Dec 16th, and then the virus definition for that day was downloaded.
At the time, the Avast! was set so the program updates did not install automatically, but the virus updates did.

The 4.8.1368 version of Avast! was uninstalled, and the previous version (4.8.1356) was installed, but that did not solve the problem.
The only way I can keep the reboots from happening is to set Avast! so both program and virus updates are not automatic.
The only opther thing that seems odd to me is that when I use the “about” screen, it shows Sep 2009 as the date for the 4.8.1368 version, which I believe was really released in November. Avast4.ini also shows Sep 2009.

I did try using the “Last Known Good” option while booting but that does not help either.

I do not have any issues with Windows or any other program updates causing spontaneous reboots.

Any ideas?

Disable Automatic Restart:
System Properties > Advanced > [Startup and Recovery]Settings
Remove the checkmark from “Automatically Restart” in the System Failure section.

Besides Google also you can use:
MS knowledgebase: http://support.microsoft.com/search/?adv=1
Free memory tester (Run it overnight): http://www.memtest86.com/
XP error explanations: http://www.xptuneup.com/xptuneup_stopmessages.htm
BSODs: http://www.techmetica.com/howto/descriptions-and-fixes-of-blue-screen-of-death-bsod-crash-errors/

The Automatic Restart in System Failure was already unchecked.

I looked at various log files in the DATA\log directory and noticed that starting on Dec 16 there are many messages in selfdef.log that say
DATE TIME Write access to file \Device\Harddiskvolume2\Program Files\Alwil Software\Avast4 xxx denied. [C:\WINNT\Explorer.exe] where xxx
is every sub-directory under the Alwil Software directory.
The PC does not have a Harddiskvolume2 that I am aware of!
The security settings for Alwil Software and all directories below that are “full control”.

I have another system (laptop) that was not online for quite a while until Dec 18, 2009.
When I brought it back online, it did update both the program and virus definitions properly without doing any extraneous reboots.
But I just looked in its sefldef.log file and starting on Dec 18 similar “write access denied” messages occur, except they are to \Device\Harddiskvolume1…

Weird, huh?