Hi Maurizio,
Thanks a lot for addressing this and moreover for the windows installer :).
Postgresql 8.3 will be supported in the next versions of OpenACS and .LRN (5.5, 2.5).
It would be nice if all the packages included in .LRN has support for Oracle, indeed. Most of us use Postgresql and there's very few people using Oracle who contribute to the community, I guess that's the reason.
Although, the most important packages support Oracle (core and dotlrn-all, see http://www.openacs.org/xowiki/Aliases_at_CVS for details). dotln-extras packages like views, survey and chat provide support for Oracle too (from the top of my head).