http://wordpress.org/tags/mod_security
Myself, as well as others, have provided info in other threads on how to deal with mod_security. be advised though, that later versions of mod_sec cannot be disabled via any .htaccess tinkering, so you may end up needing to talk to your host.
Honestly though, if your host is blocking logging into the wp-admin with over-zealous mod_sec rules .. chances are they dont know what they're doing, and you may want to look into another host (especially if they cant tweak their rules).
Im a huge fan of mod_security, but thats a tad much.