in the backups thread, there is a post on what to delete to free up space for games. basically, for halo if that's what your talking about. if you want to use the trainer just for mulitplayer maps, you can just delete all the campaign maps. the a10.map, a20.map ect. also, you can take out the /Xdemos folder and everything inside the /Bink folder accept the intro.wmv i believe. but check the backup thread. lots of info there.
Too much space