The easiest way (at least for me) to downgrade is to use the manual "upgrade method" with an archive of your previous wordpress version. (Whatever you had before trying to upgrade.) Download here.
Basically do the same thing you did to upgrade to 2.8. replace the wp-include and wp-admin folders, (and whatever you replaced in the wp-content folder.)
run the upgrade.php http://yoursite.com/wp-admin/upgrade.php (or wherever your blog is located.)
And that should be it! :D
Hope this helps.