Use classes outside template

Hello,

I’d like to know how to use wp classes (like query_posts, get_post_meta …) outside my template : I open a popup and want to get custom fields values in a simple way (without using the get parameters for more security)… Which files do i have to include ? I tried include(‘classes.php’) but it doesn’t seem to work…

Please help

Thanks in advance

(excuse my poor english)