Having gotten my production site running on XAMPP a few months ago (its fine - you just have to tune mysql settings), I have noticed the dreaded Apache/windows memory leak. The forums suggest that certain combinations of apache/mysql/php are stable, but the package I downloaded, with apache 2.2.3, mysql 5.0.27 and php 5.2.0 doesn't seem to be one of them.
600MB with no active connections during a quiet period is definitely not right.
The packages should reflect the info from the forums e.g. http://moodle.org/mod/forum/discuss.php?d=89143