It is impossbile to store any Xbox data of any sort(game saves, DLC, demos, etc) on your computer. You can store music, videos, and pictures on your computer and stream those to your Xbox 360, but that is all. For more info on how to do that, visit the following
page:
http://support.xbox.com/en-us/pages/xbox-360/how-to/xbox-360-as-media-player/media.aspx.
If you need more room for downloads and such, you can purchase the 250GB hard drive for Xbox 360, or purchase a USB drive. Xbox 360's support USB drives that store anywhere between 1 and 16GBs of storage. For more info on Xbox storage, visit the following
page: http://www.xbox.com/en-US/storage