I've used avast since version 5 without any incidents.
But this year, most noticeably the past couple months, whenever avast is updating itself behind the scenes, Windows will deadlock for maybe 20 seconds. Happens at least once a day, sometimes twice.
Same PC, same Windows XP, same configuration I've used since Avast 5.
It started happening after the last forced program upgrade after the start of the year.
I know it is Avast causing the deadlock because right after it finishes and returns control, Outpost firewall will immediately alert me that parts of Avast have changed.
2015-May-8: Now fixed from the last program upgrade