I solved the problem like this:

As my license for Norton was due last week, I removed Norton NIS2005 with
Norton's removal tool and performed some additional steps according to 'Ask
Dave' website and installed another vendor's commercial complete antivirus
and internet security product ( not any of those that was recommended in this
thread ). Now outlook works again for every XP account without having to boot.

So, I suppose it was Norton that caused the problem, and not april patches
from Microsoft. Or let's say, I don't want to bother arguing about the
possible candidate of causing the problem, because everything works for me
now.

I'll send a link of this thread to my email operator administrators, in case
they'd encounter a similar customer like me.

br: Kimmo