It depends on how you installed it and how you uninstall it, but you never ever have to reformat your machine. Basically, undelete the aolserver binary directory (usually under /usr/local or /usr/lib/) and the aolserver server pages directory (e.g. /var/www/openacs), the aolserver start script (if there is one) in /etc/init.d and the start and stop scripts. That's all. Good bye.