Please try to run "AvEmUpdate.exe /debug" from Avast program folder as an administrator and copy output.
Prior to your response I tried "AvEmUpdate.exe /Run /TN "Avast Emergency Update" and that seems to have set the update to build version .249 and so and so far it is working. My friend who successfully updated Avast on his XP earlier is now having a different problem. After Avast is running for about 10 minutes, it will lock his computer so badly he has to power it off to regain control. Once it is up and running again, the same lock up scenario will happen again after a few minutes.
There is definitely a problem...