I have to admit I was wrong.

I solved my problem few days ago. I think the problem was c:\windows\temp directory where avast! saves/deletes some temporary files during the definition update. As a user I didn't have enough rights to this directory (I was even unable to view the content of it). I solved this by moving the XP's temp directory to the shared folder. After that I've had no problems.

This problem seems to have nothing to do with c:\windows\temp directory. I still have the same problem. I didn’t know this because I was using avast with “Ask when update is available” setting for a while (it works just fine). Now I’m using the setting “Automatic” and administrator account and my computer has freezed three times a row also.
I don’t know anymore. I’ve tried almost everything. Maybe it’s related to XP’s language version somehow…? I’m using a finnish version of XP Home.

– Jeccu –