sarnia,
This is a long shot, but what updates have you installed since upgrading to Window XP? Microsoft has released several updates (announced in these security bulletins: MS03-004, MS03-015, MS03-020, MS03-032, MS03-040, and MS03-048) that will break window.showHelp( ) if the updated Help Control found in patch 811630 has not been installed.
I have doubts as to whether this could be the cause of your problem, among others being that the issue raised by MS appears to be related to displaying help files not loading them. You didn't mention under what circumstances the error occurs. I don't know exactly what the Help Control does or doesn't do, or how it reports the problem when it fails due to the missing patch. Last, I don't know how avast! reacts in that situation nor what error it logs as a result. Thus, this is pure speculation on my part, but not a total leap of faith for me, to see a possible connection between your problem and the update issue.
Regards,
Hornus