query_post not working properly

i am using this method to list all games tagged in system and has post type game.
I t is not working properly because it lists posts from all post types containing tag=system .
After spending a lot of time i found that only one query is processed at a time not both.
query_posts( 'tag=system&post_type=game' );
I am new to wordpress.so plz help me..
Actually i want to list posts in a tag archive by post type.

For example,
Games tagged in ‘system’
//posts
posts tagged in ‘system’
Plz.. help me..