You sure it's INF_Core.int and not INF_Core.u? In any case version mismatches of core files are usually caused by joining a PsyTex server without PsyTex Inf or vice-versa. Install PsyTex as described in the link for the perfect Inf install in one of my previous posts. Note that since you run Vista you'll have to make a manual installation - not as difficult as it sounds, you just need to copy the right files to the right place in the right order.
The downloading stuff from server is usual when joining COOP servers, since they run mission packs which are not included in standard Inf - just auto-download it; you still can download the entire packs later if you want to use them for single-player.