I've been using Windows Live Writer for a while now, but somehow it never worked properly from behind a firewall. Moreover, Aviva Solution's weblog is based on Sharepoint 2007, and WLW did not support this type of blog, up until now...
The Live! team has released beta 2 of Windows Live Writer. And obviously, this post has been written with this new beta from behind a firewall! I haven't been able to test with the MOSS blog (because WLW doesn't seem to like an ISA server in between), but I'm sure going to try real soon.
The original article that pointed me to this new beta also mentions a soon-to-arrive add-on that adds additional blogging features to MOSS 2007.
You can download beta 2 here. A note to the Dutch readers, make sure you tweak your IE language settings in such a way that the English language is on top the list. If you don't, you'll end up at the Dutch version of writer.live.com which does not host the new WLV version.