Today I will test buddypress on WP 3.0 so I install new WP and buddypress on my localhost testing system, after finish install i get HTTP ERROR 404 NOT FOUND so I searching for the solution.

In my case, fix it by:

change http.conf to use mod rewrite by uncomment line

#LoadModule rewrite_module modules/

then change "AllowOverride None" to "AllowOverride All"

# AllowOverride controls what directives may be placed in .htaccess files.
# It can be "All", "None", or any combination of the keywords:
# Options FileInfo AuthConfig Limit
AllowOverride All

after change apache config you have to restart apache.. done

another reason that may happen is wordpress can't write .htaccess in wordpress directory when you change permalink in wordpress setting

solution is change .htaccess file permission from 644 to 666 then change permalink setting from wordpress dashboard, after that change file permission back to 644.

wordpress will write new access rule to .htaccess according to your setting correctly