You should post .htaccess for us to se. But anyway. You should exclude "admin" to be directed to index.php
something like this
RewriteEngine on
RewriteCond %{HTTP_HOST} ^domain\.com$ [NC]
RewriteRule ^(.*)$ http://www.domain.com/$1 [R=301,L]
RewriteCond %{QUERY_STRING} ^(ACT=.*)$ [NC]
RewriteRule ^(.*)$ index.php?%1 [L]
RewriteCond $1 !^(admin|index.php) [NC]
RewriteRule ^(.*) /index.php?/$1 [L]