I have just spent over 3 hours on this.

I am too still running Windows 2000.
Hence I a few moths ago I locked Avast not to update the program from 5.1.889 but just the definitions which I understood to be still compatible for the foreseeable future.
Late yesterday I got the pop-up box that an Avast file has caused an error and will be shut down by Windows with the result no Avast services were running.
I assumed an issue with my system and that the fix would be to uninstall, clean the registry and reload.
I found what I thought to be the installation package for 5.1.889 but it turned out to be for 6.0.1203.
This installed but with an error about mail scanning.
I then found the 5.1.889 installation package with of course virus definitions dated several moths ago. This installed fine but again the error stopping all Avast services once automatic updates stated.
Hence I downloaded the manual definition update file, stripped the installation, rebooted and reloaded 5.1.889 again followed by the update file. This reported to have installed but the main screen showed the old virus definition date. Hence I rebooted.
Again Avast was not running and clicking either Fix or Start services on the main screen did nothing.
Temporarily I am back to 6.0.1203 so definition file only about a month out of date and I have locked this not to update anything.
In between this of course I scanned my PC to check I had not acquired a gremlin causing the problem but the scans found nothing.
Once the next bug free definition file is posted I assume I can go back to 5.1.889 but should I let it update the definitions automatically or download the update file to update manually?
Also I have told a friend also running Windows 2000 not to turn on his PC as if Avast locks up I will have to sort him out too as he is not very computer literate.
Once the next definitions are released can he just allow them to run without problem or will I have to run the manual file so the file with this bug in it is skipped?